[DMetalKong]

In my opinion, trying to write specialized apps for various systems like iPhones, PDAs, etc. is overkill. Most likely what you write for one system will have to be re-written for another device. However, you could do a multi-platform solution would be by writing a web-based app, which only needs a browser to function. As long as the markup is simple, the functionality should remain the same across a wide number of platforms. Granted, there are probably some specialized functions of mobile devices that a web app doesn't cover (Does the iPhone allow file forms in the browser, for stuff like uploading pictures from the phone?), but the simplicity of a web-based app is nice. Just my $0.02.