How can I develop mobile applications using HTML5?

You may also want to try MoSync (www.mosync.com) for it, the advantage of MoSync is - it's truly cross platform and it let's you write hybrid applications as well (using C++ and HTML5) and being cross platform if you write a plugin it goes on to every platform, which would mean we don't have to write separate plugins in Java, Object C, Dot Net etc. as it is the case on PhoneGap.

I wrote a tutorial to make a HTML5 twitter client app in a minute or so using MoSync and Sencha Touch and it may help you.