FWIW t-mobile offers port 80 traffic for free (free as in uses your minutes ... i think). If you need ssl, pop3, telnet, ftp, or any other non port 80 services you need the data plan.
i think verizon does too?
Anyway ... i've had t-mobile for a while and am fairly satisfied. I've never been charged for roamingm, even when I've made calls on other provider's towers.
If I left t-mobile I'd probably go with cingular.