Is it hard to start a PhD programme after leaving to work in industry?
Is it hard to resume education after working in industry (assuming 'ideal' conditions such as work related to chosen field, good master project from good university etc.)?
In short: NO.
I did exactly that and I think that was a great thing. Being in computer science, this gave me solid experience in software engineering and pragmatics of "real-world" solutions. The benefits I feel till now (after completing my doctoral studies and working as a relatively senior post-doc (3+ years after thesis defence) are these: 1) good feeling for what "societal" problems are and how does my research translate to practice (at least hypothetically, but in a plausible way) and 2) allows me to be involved in applied projects in any role from low-level programmer, through technical lead to project manager. All this is vital in writing project proposals and project execution. In a nutshell: experience in industry, when used wisely, can give you an "entrepreneurial" attitude, which definitely is an advantage over students who plunge to doctoral studies right after completing their master's degree. The only slight downside is that you might end up as one of the oldest PhD students in the group. But I never perceived it as a problem.
Finally, all the above applies to experience in European context (in particular: DE, NL, BE). I have no clue about the cultural issues regarding your question in other parts of the world.
Later edit: When it comes to the emotional and lifestyle part of the decision, of course there are issues to consider. Going from an industrial position to doctoral studies is almost always a financial downgrade. Perhaps more in countries where a PhD student has a student status (US, UK), than in places which treat PhD students as university (public) employees (DE, NL), salaries tend to be higher in the latter. My own attitude, however, was this: since at that point I did not have kids yet, I always thought that should the life demand more money, or when I won't like the academic life, with the sound experience from industry I shouldn't have a problem going back any time. This definitely took a huge amount of pressure from my shoulders while pursuing my PhD, since I did not worry about my future (in career, or financial terms) - unlike my "purely academic" peers. Even till now, I feel confident (perhaps I fool myself) that should the academic path not work out in the next few years, it's not going to be the end of the world for me (again unlike for some of my peers). To sum up: with the confidence that I am fit for industrial career, I can pursue my passion in academia, rather being under pressure to produce. So I would add this as yet another benefit.
P.S. To a more extreme note: I can point fingers to at least two people who after a long career in industry embarked on doctoral studies in their 50s and became successful researchers in their fields afterwards. Similarly, there are many people who after completing their PhD went to industry for awhile (5-10 years) and later came back to academia - though that feat seems to be harder to manage than the previous one. So everything is possible...
Even later edit (8 years later): I do no longer work in academia, at certain point 1) my family had enough of moving, 2) I ended up in a region with very competitive universities, 3) since I probably did not belong to the top notch, but let's say just slightly below, at certain point my funding dried out and I did not get a suitable professorship in that region. This led me to first seek job in industry again and later to start my own company/statr-up trying to capitalize on all the scientific knowledge I collected. Retrospectively, my career as an engineer before joining academia turned out to be a great asset once again. I could claim very solid history in my CV and I was immediately hired to senior positions. Also, the academic training in research creativity turned out to be a very useful asset when I started my own company later on. All in all, I am happy about this career path. Maybe this later edit will inspire others too... Good luck!
From an admissions standpoint getting into graduate school after a few years in industry is not much harder than going straight from undergraduate. Some undergraduate programs in the US offer 4+1 programs which make getting into a Masters program much easier, which in turn makes getting into a PhD program easier.
Becoming a student again after working in industry can be a hard emotional and psychological shift. Your pay will be cut in around half (maybe even more). In the US you will likely be expected to complete some more coursework. For some it is hard to get back into the coursework mindset after a few years absence: Assuming a linear scale, it is the difficulties you face every fall as a returning student multiplied by 10 (or so). It might even be an exponential scale. It can be hard. Being a grad student is nothing like being an undergraduate student nor is it like working in industry. The job of a grad student definitely doesn't end at 5 pm and tends not to have 2+ weeks of vacation. There is also considerable uncertainty associated with long term job prospects that are absent in industry work.
That said, those who love academics often find them selves out of sorts when working outside of academia and relish returning "home".
One thing that hasn't been mentioned in the answers to this question pertain to recommendation letters, which are a key and important part of the application process. If you decide to leave academia for an extended period (more than a year or two), it would be wise to keep in touch with your advisor and some other professors so you don't surprise them with a "could write me a letter of recommendation for a PhD program" many years down the road. They need to be able to comment explicitly on your potential to do research, and (for U.S. universities) you'll need three solid letters to be competitive.
I might suggest asking for letters of recommendation now, and plan on contacting your letter-writers later to ask them to look back over the letters, and update if necessary. This way they aren't scratching their heads trying to (1) remember you, and (2) write a quality letter a few years after they knew you and your research.
By the way, I started a Master's/PhD program about 15 years after I got my Bachelor's (and another Master's in a different field), and finding appropriate and relevant letter-writers did prove a bit challenging. I ended up getting a letter from my then-boss (science department chair at the high school where I taught physics), a professor of education from my previous Master's degree, and a physics professor with whom I had been taking some "physics pedagogy" classes (and all for degree programs in computer engineering!). In the end, I successfully got accepted into many of the schools I applied to, but I'm sure my case would have been helped if I had letters that commented more specifically on my ability to research.