Since I'm new here I figure I gotta be careful with recommendations, but I had a really good experience with a website
www.firstpcs.com
Got a Mitsubishi web-browsing capable phone for cheap, and the AT&T plans seemed pretty good--definitely less than your $50 a month for many of the plans. They also had an Ericcson webphone for even less, like $60 bucks plus some kind of $50 credit towards other purchases.
Dunno. Webphone is the most exciting thing I've bought lately. Came here for deals on OTHER toys.
(edit)
Thought it was obvious, but the URL is
http://www.firstpcs.com