Hi there,
Just got my Treo yesterday and I'm very new to SMS myself.
Here are a few things I've figured out:
You can use SMS to send to regular internet addresses (joeblow@yahoo.com) by entering 121 for your SMS email center number (Part of your SMS preferences). 121 is for Cingular, I believe, 500 is for Voicestream.
You can also receive internet messages if your friends send an email to
xxxxxxxxxx@mobile.mycingular.com (the x's are your 9-digit number). I've tried sending messages to myself this way and it works.
Not too sure about who else you can SMS though.
Have fun!