I know somewhere around the forums, a user has made a listing of the sms email addresses for the carriers. You can also (on some sites) go to the website of the carrier and get that address. I remember some but not all the email address.
(replace sprintpcs with nextel for nextel users)
Those are the ones I remember. Hope it helps. I usually program the email address into my address book under the name of the person so I don't have to remember, I just select email the person and that's it. Works well for me, I even get pictures too. Not bad for a BB 7510 from Nextel!