Rhodes by Rhomobile is a Ruby framework for building cross-platform phone applications. It allows you to build a single application that works on all major smartphones: iPhone, BlackBerry, Windows Mobile, Symbian and Android. (The only obvious omissions seem to be OpenMoko and PalmOS/webOS, but all the phones you listed are supported.)
The way Rhodes works, is that you write your application in Ruby and your UI in HTML. A Ruby implementation, the Rhodes framework itself, your application scripts and your HTML files then get packaged up into what looks to the phone's operating system like a single native application. Rhodes then runs a webserver inside of the phone and serves the application from there, using the phone's builtin web browser UI component and a JavaScript UI library for making the web app look like a native app. (E.g. iUI for the iPhone.)
There was a nice introduction to Rhodes (with live coding) by the Rhomobile CEO at the Mountain West RubyConf 2009, the video is available at the Confreaks website.