Yes it is!

http://www.blackberryforums.com/showthread.php?t=2019

The instructions vary by carrier, but this article does have a section
for Cingular and what settings and commands need to be applied.

It's possible.  Last month I got about 1,000 emails or so, and topped
out at 4.1MB.  And I know this month's usage is going to be...
interesting... because today a fellow sysadmin at work erroneously fired
up a server due to be retired, whose mail process had been down for a
while.  As a result, our blackberry-equipped staff (myself included) got
hit with 387 server-generated e-mails in just 5 minutes before he
realized what happened and shut down the process.

Bottom line is, a Blackberry's core application IS e-mail, and e-mail is
something which you have little to no control over as far as what you
receive.  As such, for the extra 10 bucks, I'd get the unlimited plan if
I were you.  And considering that you seem interested in using the phone
to tether your laptop you'll definitely want the unlimited plan, as 4MB
isn't going to be enough for both e-mail and a modem connection.