I'm an iPhone 4 owner, and I came REALLY close to getting a Titan a couple of months ago. I left WM for iPhone in 2008 simply because MS wasn't being competitive. If WP7 had come out in 2008, I'd probably be using WP now.
There's several things that stopped me from switching, but two I think are worth noting here.
One, I've had a iPhone since 2008. I bought an iPad in 2010 and 2011. I'll buy one in 2012. My wife, whom is a die-hard Apple hater loved my iPhone 3G so much that she made me get her one on AT&T and we rode out the rest of her contract on Sprint with that phone unused. Suffice to say, I'm pretty well invested in the Apple ecosystem and the thought of having to rebut or find substitutions for applications I use wasn't a good thought.
But beyond THAT, the most important reason I didn't switch, was that I couldn't convince myself that WP7 did anything significantly better than an iPhone. Every review seems to talk about the great social integration. I don't use that crap. I have twitter and facebook. I tweet maybe once a week and log in to Facebook maybe once every 6 months. I just simply don't care and I don't want it to be a part of my life.
When I tried to examine the WP platform without taking in to account the social integration, it seemed like WP7 just didn't have anything. So, Apple gets my business for another year.
But this is great marketing and they're totally right. For the vast majority of people that use social sites very often, WP7 is hands-down the champion.