If you are looking for a good platform to design applications for Iphone, Ipad, Android and Windows phone, you should definitely consider a browser based approach using JQuery Mobile. Jquery Mobile has very good user interface and page transition mechanisms.
Jquery Mobile 1.2. 0 Alpha has been released.
Jquery Mobile Docs has excellent documentation
Quick start guide is a good place to start.
Eric Sarrion has an excellent The jQuery Mobile tutorial which has lot of useful information about how to construct basic pages, buttons, sliders etc.
10 handy jQuery mobile tips and snippets to get you started has excellent tips.
You can also look at 50 jQuery Mobile Development Tips.