Cingular's $20 data plan is something you add onto an existing account. Standalone, they want to charge much more for Blackberry or data-only plans. The cheapest way to get a Treo 600 going would be $20 for unlimited data on top of the $29.99 calling plan. Things get better if you've already got a Cingular phone. You can get a FamilyTalk plan, getting plenty of minutes for one phone, and pay $20 for unlimited data for the second phone. I don't see any easy way to buy a data-only plan.

T-Mobile offers a $30/month unlimited Internet plan that is what you really want, pricewise, but you have coverage issues.

Verizon wants $50/month, either for a PDA or a Blackberry.

So, when you say "T-Mobile doesn't work", do you mean zero coverage or just poor coverage? It may be the case that the data service is usable, even if/when voice service sucks. You're getting best-effort, which may be good enough for the price you want to pay.