MMS for the iPhone 3G on at&t does work. I've been able to send and receive messages directly from the iPhone 3G using the following instructions.
USE AT YOUR OWN RISK
1) This is experimental. The instructions might not work or might cause your phone to stop functioning completely.
2) at&t does not support this officially yet. There have been some reports of problems associated with enabling MMS on at&t.
3) The information is provided here for educational and informational purposes only. It should not be used on any phone.
This requires an iPhone 3G loaded with the iPhone OS 3.0 and 3.0 SDK installed onto a Mac with Leopard 10.5.
Step 1) Prepare the ATT_US.ipcc
Follow steps 1 - 7 from the instructions
here.
1) Under Root in carrier.plist add:
AllowEDGEEditing Boolean Checked
AllowMMSCEditing Boolean Checked
AllowMMSEditing Boolean Checked
2) Add the following:
3) Under root in carrier.plist also add:
mms_proxy String 066.209.011.032
mms_proxy_port String 80
mmsc String
http://mmsc.cingular.com
Now follow Steps 9 - 12 in
the instructions to finish preparing the ATT_US.ipcc.
Step 2) Enable MMS on your account at at&t.
Removed content related to enabling MMS due to problems. I have been looking at SwirlyMMS threads to get an idea of how to enable it, but no solution so far.
I have not tested this with tethering. I will let someone do that if they want to.