Last time I checked, you did not need a special plan. Rumors suggest
that T-Mobile may make some changes, which _might_ require a $5/month
data plan. You're able to do standard HTTP, as well as SMTP, POP3 and
IMAP email on any current voice plan.

Note that if you have an _old_ plan, it's possible your account hasn't
been updated, so you should check in advance to see if it works. The
APN you'll use is "wap.voicestream.com"

One thing to remember is, despite marketing peoples' attempts to make
a distinction, there's really no difference between using your phone's
built in apps, your PDA, or your desktop computer. None of the phone
networks that I am aware of can actually tell the difference. Except,
of course, that it's hard to use a gigabyte a month on a J2ME phone :)