T-Mobile, last I checked, offered $20/month unlimited data, when "attached" to a voice plan. I have their service, it works well in places that have coverage. GPRS speeds over by T610 phone are 3-4 Kbytes/second. My understanding is that you can get $30/month data plan without a voice plan.
I'm moving to Cingular, which has a $40 unlimited plan (but you have to call to get Pay-Per-Use off your bill). They presumably have EDGE and also coverage to my new house.
I use T-Mobile to download sometimes over 20 Megabytes per day in RSS feeds and podcasts and videocasts using FeederReader directly to my Pocket PC.
Greg Smith
Author, FeederReader - The Pocket PC RSS reader and podcatcher
Catches video, too!
www.FeederReader.com - Download on the road.