The 6.0 carrier file is all that is needed to get MMS and tethering working on OS 3.0 and 3.0.1. The APN changer you reference should not be needed.guys remember that 3.1 disables tethering no matter what carrier file you have. However, 3.0/3.0.1 with carrier 6.0 and the Profile from this website makes tethering available
access this website from your iphone safari browser:
and install a custom profile from AT&T that will enable tethering.
Note: make sure you update to carrier 6.0 first.
I got MMS and Tethering working!