This is from XDA - ALL credit to them I am an idiot. But you do NOT have to root to get tethering.
Here is a step by step:
Menu - Settings - Wireless & NetworksMobile Networks - Access Point Names - Menu - New APN, Enter these settings:
Name: AT&T
APN: wap.cingular
Proxy: leave blank
Port: leave blank
Username:WAP@CINGULARGPRS.COM
Password:CINGULAR1
Server: leave blank
MMSC:
http://mmsc.cingular.com
MMS Proxy: wireless.cingular.com
MMS Port: 80
MMS Protocol: WAP 2.0
MCC:310
MNC:410
Authentication Type: leave blank
APN Type: leave blank
Menu - Save, then select New APN (AT&T) You should see a green dot next to it, and it should be labeled under AT&T as wap.cingular. Go back to home screen and check internet connection is active. Once you verified internet is working, power off phone, then power on. Phone should still have active internet connection, if it does, you are done!
Here's the link.
Tethering on Unlimited Plan with Atrix - xda-developers