When the iPhone came out, it was enough of an improvement for me to switch, and I've been glad I did ever since. Now that the Pre is coming out, I have to say that I'm impressed. But I question if it's enough to switch again? Like Going from VHS > DVD > BluRay. Yeah, each format brings it's improvements, but is the improvement from iPhone to Pre enough to keep jumping. My first gen iPhone contract ends on Dec of this year, and I think I'll continue to stay on the iPhone train for the next two years at least.
Although, I've mostly been using Gmail and GCal for my scheduling. So at least that portion of my life would switch over easily. And iTunes integration for Pre would make the music element easier too.