This sounds like a decent deal: Sign up with one of Earthlink's 2-year voice+data plans (start at $69.90/month including unlimited monthly data) and get a Treo 650 for $199. More details here.
Caveat: I've no personal experience with Earthlink as a cellular provider, and I'm sure they're reselling minutes on somebody else's networks, but I thought I'd pass on the info about the cheap Treo 650.
I have the Earthlink Treo 650, and I've been reasonably happy with it. The service is through Sprint and Verizon, but I only have data service when I'm in a Sprint coverage area.
I have heard some reports that your primary (and thus presumably data) service provider is determined by where you live, but I have no idea whether that's true.
I have had a lot of issues where the phone sends calls straight to voicemail when the data service is in use. This used to happen once in a while, but in the last week or so it's happening more often than not. I have no idea whether this is the result of a worsening defect with my particular phone, transient misconfigurations somewhere, or what.