Ask Engadget: Best wireless data plans?

A couple of months ago we wanted to hear your worst (or best, as the case may be) cellphone horror stories, this week we want to turn things around a little bit, with a question from Andrew, who wants to know which carrier has the best wireless data plan?

I've been with T-Mobile for a while and love my Nokia 3650, but I'm looking to move up to either a Treo or a
Pocket PC Phone. I would love to know which of the wireless providers that are subsidizing these phones has the best
for use with smartphone. Which carrier offers the cheapest data plan?

Sprint's $15 a month flat-rate seems like the one to beat, but we've heard of T-Mobile and even AT&T Wireless coughing up nice flat-rate deals if you lean on them a little bit. Any advice for your fellow reader?