Another plug for the open development Google Android series of phones. Resist the Fruit.
http://androinica.com/2009/06/11/appcelerator-offers-one-stop-iphoneandroid-development-tool/
I've considered it, but I've already done most of the translation to Java for Android.
Ehh, it's a little short on details... It says it uses "HTML, JavaScript, and CSS to build native apps for Android and iPhone". I think this is more of a "native" web application type of thing, like the early iPhone apps.. but cross platform.