First thing you should decide is if you want an iPhone, Android, or Windows Phone 7. I doubt you'd want a Blackberry, it's mostly a business oriented device still.
I can't speak much about WP7, I don't really follow it. My impression is it's a locked down OS like iPhone, but with only a fraction of the apps.
iPhone is nice because it has tons of apps, is very polished and smooth. But it's a walled garden and there are less freedoms with it.
Android is nice for almost opposite reasons. It's very open, you have lots of choices in hardware configurations and styles, and even without rooting (hack to give yourself full access) you can still do so much with the OS. And Android has a lot of apps too. But it's not quite as smooth and polished and because of the big fragmentation in different versions of Android, different hardware configurations, and different custom UIs, the app experience is not uniform and sometimes stuff just doesn't always work like it should.
As a tech person I love Android because it allows me to do so much with it. While Apple's philosophy is to make technology hidden to the end user (boring).
If you decide on a Android phone, then you still have a big challenge of deciding which Android phone, as there are tons of them. But I'd probably look at only phones with dual core processors, that should help narrow down your focus.
Of course you'll want to decide on a carrier too, probably before trying to figure out which phone you want.