Is it common and acceptable for a postdoc to leave before contract ends?
Usually, a postdoc is a normal contract between an employer and an employee, and as such, terminating it earlier than its date depends on the contract and/or the country. It should be normally specified on the contract, otherwise the laws of the country should apply. For instance, in the UK, I have had a notice of 1 month, in Italy, it was 3 months.
It is normally acceptable for a postdoc to leave before the end of the contract, although it depends on the situation. If the employer does not provide any guarantee for further employment, and if the postdoc has found another offer, starting earlier than the end of the current contract, then that's the rule of the game. If the postdoc has found a much better offer (for instance, a permanent position), then that's also the rule of the game. If personal reasons are involved (going with a partner, going back to home country, etc), that's quite fair.
By acceptable, I mean that the employer should normally not make any problem (assuming the legal obligations are fulfilled), and might even be supportive in the end of the contract. A case that might not be acceptable is to leave in the middle of a contract, breaking some work commitment (e.g., an experiment to run) for a reason that might not appear very strong. But in the end, it depends a lot on the relationship between the postdoc and the employer. The point to remember is that Academia is a small world, and that in general, it is worth keeping good relationship with former employers. In doubt, talk with your current employer, or with a mentor at the place you're working at to know what the rule normally is.
To add to previous answers: if your postdoc includes teaching, you should finish out the academic year, or at the very least finish the current term, and give your department chair as much notice as possible, so that they can find a replacement or adjust teaching assignments.
In my experience (I'm a postdoc now, and hence also know a few other postdocs) leaving before the end of a contract is perfectly normal. The nature of postdoctoral work is that you have to be ready to take the next opportunity when it arises - other people certainly aren't going to make sure the timing lines up nicely for you! Of course if you leave very early in the contract, that might annoy people, but assuming you're reasonably sensible, everything should be fine. Remember that the people employing you have either been through the same process, or observed it over the years1.
As for how early the notice should be, that depends on the contract and the labour laws where you are. There may be certain legal minimums that have to be observed, but the figure seems to normally be 2-3 weeks. Of course your employer may let you out early by agreement, but as your employer is probably technically the University, don't expect a bureaucracy to be particularly flexible.
- Little side note, I gather this might be a bit different in the US, though currently in the process of changing (perhaps someone who went through the US system can add to this). A postdoc in the US has lower status that it does in Europe/Australia/New Zealand/..., so it might be the case that your employers haven't been postdocs at all.