Bob - I haven't tried AccuTerra. I was using one on my Android phone called Backcounty Navigator that I really liked. I already had the Trimble app and wasn't crazy about it. Recently Trimble acquired MyTopo and stopped all other vendors from being able to access their maps (including Backcountry Navigator). I decided to try the Trimble app again and found that the latest version was much better. The only thing I don't care for as well is their method of selecting map sections to cache (although they have a cool interface for doing it on the web and then downloading to the phone offline). I also really like the free Trimble topo app on the iPad (and Nat now has an iPhone 4S so we can use it on both hers and mine for hiking).
Brad - you're right about the Android seamless integration with all things Google. They now at least have the Gmail app on iPhone and Google Sync works pretty well but it's not the same as Android. That said, I think I'd still trade my Droid X2 for an iPhone 4S most days just because the iPhone is so much more stable and performant. Nat really loves hers.
Kathy - once you get into the smartphone world you'll never look back. Nat is definitely not a techno-geek in any way and once she got her first smartphone (which was an Android 2 years ago) you'd have to pry it out of her cold, dead hands ;)