How can I develop an iPhone app in HTML5?

Yes.

You can develop an HTML5/CSS3/Javascript app, then wrap it in PhoneGap or Apache Cordova to make it native and put it in the App Store (or Android Market...), as well as publish it on the web. You can do this with whatever tools you're comfortable with.

Using a tool like PhoneGap has many benefits over browser-based Mobile Web Apps, such as allowing you to parse iPhone contacts and access the local hardware.

It's great in combination with jQuery Mobile, but since performance is a MAJOR issue you must be very cautious to really streamline your images, CSS, and Javascript.

See this link to learn more about Mobile Frameworks.

I have seen a blog which claims that we can develop iPhone applications in HTML5. Untill then I was aware about Objective-C on Mac.

If you want to write a native iPhone application, then Objective-C is your only option. However, as the iPhone’s web browser has a lot of capabilities (e.g. offline caching), you can develop applications that run in its web browser, as opposed to natively. They can’t access all the features of the iPhone, and they are simply websites (so you need a server to serve them), but before the iPhone SDK was released, Steve Jobs himself described web apps as the way to write software for the iPhone.

Can we develop an interface with backend support application in HTML5 on iPhone? Will it be secure and scalable?

I have no idea what that means.

If HTML5 is a markup language, then how can I make conditional statements in it? Would it be via jQuery or Javascript?

Correct: JavaScript. (jQuery is just a JavaScript framework, and it’s probably a bit heavy to use on current iPhones.)

The HTML5 spec blurs the distinction between HTML and JavaScript by defining the DOM interface for the HTML elements it specifies, and defining new DOM features (e.g. offline caching) that aren’t technically part of HTML. Apple (and other people) have further blurred the term “HTML5” by using it to encompass various CSS features like animations and transforms, which you’ll probably find very useful for making web apps feel more like native apps.

A great place to start when developing an iPhone app in HTML5 is PhoneGap.

You could either go the full-blown app route by developing an iOS app using PhoneGap, or only selecting certain tools/frameworks to help build a web-based app for mobile devices.

And very much like how it works with websites, HTML is used for the structure, CSS for presentation and JavaScript for behaviour. So yes, JS is used for conditional statements, etc.