no one has windows phones... lol

 

Java/Android is generally the easiest to program for. There is a reason most uni's teach java and use android as a teaching platform.

As for WP7, I can't believe it will ever be as popular as that graph suggests.
As someone who works for a mobile app development company, I can only see android becoming stronger and stronger. Business forecasts put the Amazon kindle tab platform (android) as being a big hit this christmas.

If you really want write a mobile app for all platforms, html5 is probably the easiest cross platform system, since it can run either in a browser on all devices, or with an app wrapper.

(As for the comment about IOS running java, don't bet on it)

 

If you're not going to use html5, or html5 boilerplate you should at least take a look at the project as they have a lot of good tips.
http://html5boilerplate.com


This post on stack overflow has some nice information in it.
http://stackoverflow.com/questions/5063117/choosing-mobile-web-html5-framework


Another popular framework that wasn't mentioned on the above SO post I thought I'd mention. (If you're going to use bootstrap for mobile devices, make sure you're using responsive)
http://twitter.github.com/bootstrap


As a side note, I would just like to mention that the HTML5 specification is still a Working Draft. Not all browsers implement HTML5 and CSS3 fully or have the same features implemented. And just to make life more interesting, some people still enjoy using IE 6 and in my opinion, anyone who still uses IE 6 is a masochist.