One of the problems i've noticed is that there are literally dozens of custom/hacked .ipcc's and mobileconfig files floating around the forums, all to enable tethering. And all have different settings it seems.
There really is nothing different between .ipcc's and mobileconfigs, other than mobileconfigs can be installed via the iPhone itself. The first mobileconfig link posted in the first page of this thread has this in its APN settings:
<key>apn</key>
<string>wap.cingular</string>
<key>type-mask</key>
<integer>-2</integer>
<key>password</key>
<data>Q0lOR1VMQVIx</data>
<key>username</key>
<string>WAP@CINGULARGPRS.COM</string>
If I understand correctly, the "Username" string HAS to be
WAP@CINGULAR.GPRS.COM, as shown above in the first mobileconfig file on the first page of this thread.
I know that when I stick my sim card from my 3G into an aircard, I use the wap.cingular gateway with the
WAP@CINGULARGPRS.COM username and the "CINGULAR1" password, which is the typical APN used for iPhone data. With this setting, I never get charged with using my iPhone data plan in an AirCard - it looks the exact same as me using the data through the iPhone itself. I also use the CINGULAR1 password, which, again, is the standard password. I don't know what's up with the random letters in the password string above.
The next step is the "type-mask" string. With a string of -2, i THINK that's what tells it what services need to use the wap.cingular APN - in this case, -2 could mean any combination of VVM, Data, Tethering, MMS, what have you. I am not quite sure what the correct combination is, but if MMS never works for people with a -2 setting, then that probably has to be changed.
The last thing is, a 2nd APN is present under the wap.cingular one, which is the acds.voicemail one, which benm.at fixed so that visual voicemail would not be broken. That is correct.
What this boils down to is making sure that the settings in the 1st APN have the correct wap.cingular settings, and then figuring out the correct type-mask to enable MMS/Tethering/Data all through the same gateway. If people are tethering and the data being used is not showing as the normal "wap.cingular" gateway, the APN is most likely not correct in whatever mobileconfig or .ipcc that they are using. There are a few mobileconfigs and ipcc's floating around with no credentials in the username/password strings, so that may be a problem and could be a reason why when some people tether, it's not being correctly routed through wap.cingular, thus not really looking like it's normal iPhone data usage. Until the jailbreak for 3.0 comes out, i'll be unavailable to fool around with this...but if anyone else wants to, feel free. I could be totally off here as well.