As mentioned in the post above yours, T-mobile is much cheaper. It's a $20 addition to your cell phone plan. I don't know how much it is standalone. But for comparison, their wi-fi (HotSpot) plan is $20 for T-mobile cell phone subscribers, and $30 standalone.
Both plans above are "all-you-can-eat" internet. The "internet through cell phone" (GPRS) plan comes in 2 flavors, one called VPN (I assume you get a public routable IP address with this one) and one with Corporate Email, presumably something like Blackberry. Since "Corporate Email" doesn't interest me one bit, I don't know anything more about it.
GPRS has a max speed of 56 kb/s, the same as a land-line analog modem. However this is an all digital connection. However it is unfortunately slower than a landline analog connection, with tremendous lag. Using it on my laptop through a USB cable connection to my Motorola cell phone, I could surf without issues. I used Galeon (most folks use Firefox) and keep multiple requests open in different tabs. So the different pages download while I read one page. It's usable, no issues. It is reported slower than the Sprint one, but check out the price! When T-mobile goes to EDGE, I'll get speeds comparable to the 1xRTT (I hope).