PhoneGap is good cross-platform, but there are several hurdles to overcome. You'll need a Mac to program for the Apple platform, and will need to download their Xcode SDK software; you'll need to familiarize yourself with how it works.
If you're building for Android, you'll want to download Eclipse and learn how to use that environment.
And so on with all the other platforms. It's hard enough learning to handle just these two environments that I have no taste to learn any new ones soon!
I've already answered a similar question about PhoneGap on my site (see my sig). Look up the link at top of the page regarding explanation of PhoneGap for the Layman...