Coolest feature: Provides an automagic, one-click way to dial up all of those Facebook friends.
Observation 1 (I know, I’m repeating myself): Don’t “friend” people who you don’t want to have the ability to have an automagic, one-click way to dail your phone number.
Observation 2: In my opinion, this is the best execution I’ve seen yet of a web-service optimized for the iPhone version of Safari.
More detail: Mashable.com
Later: No way is it as cool as iphone.facebook.com, but this is a clever marketing gimmick from a market niche that is infamous for its lack of clever marketing gimmicks: mobile.harpercollins.com.