Oh well, since nobody PM'd me to help me figure out the method I learned. I guess I'll post it publically and hope nobody does anything bad to their phones.
First, grab this mobileconfig file:
http://andrew.harrison.org/uploads/misc/3Tether.zip
Now edit it to replace "3services" to "wap.cingular"
Save, and then email yourself the file.
Open the email then open the attachment. Tap install. It may warn you it's not authentic but just do it anyway and it will install it.
At that point you should have tethering enabled. People have confirmed this works.
Now there's speculation that you can get MMS to work through this method too. Someone suggested this for the mobile config:
<key>MMS</key>
<dict>
<key>GroupModeEnabled</key><true/>
<key>MMSC</key><string>mmsc.cingular.com</string>
<key>Proxy</key><string>66.209.011.032</string>
</dict>
</dict>
<key>DefaultsDomainName</key>
<string>com.apple.managedCarrier</string>
</dict>
</array>
Try at your own risk, but the tethering has been confirmed from a few people.