Is that possible with T-Mobile? It's not a huge deal, but it would be nice.
Yes, and it's not all that hard to do. If you haven't synced your phone to your Mac yet, do so.
1) Make a snippet of whatever song you want in iTunes (non-protected file only):
1a) Choose your song and snippet thereof - Info -> Options -> choose start and stop times.
1b) Convert it to MPEG. To save space you can drop the bitrate down to 96 kpbs.
2) Move the snippet to your desktop.
3) Turn on Bluetooth on your Mac and your Phone. For the RAZR, it's Settings -> Connection -> Bluetooth link -> Handsfree, then choose yes to turn BT on.
4) Under the BT menu on the Mac, choose Send File. Find your ringtone on your desktop choose it and send it on over.
5) When it's done transferring, it will play on the phone and you'll get an option as to how to store it.
That's about all there is to it. You might have to play with the start/stop times of the snippet a little to get it just right, and best to keep it under 30 seconds.
I'd also highly recommend getting a plastic case for your RAZR, and/or always making sure your ears are scrupulously clean. The thing picks up dirt and grime like a magnet, and it's really hard to keep clean.