Here are the directions as I understand them:

1. Install threaded SMS application found here.
2. Using a registry editor app, navigate to HKEY_LOCAL_MACHINE\Security\PushRouter\Registrations\ByGenericId
3. Delete the PC10 key (folder).
4. Find the key that has a 'Path' value of "\windows\MMSBallon.exe" and copy the 'GenId' data value. It should look similar but not identical to 80409F46E414C801.
5. Navigate to HKEY_LOCAL_MACHINE\Security\PushRouter\Registrations\ByCTAndAppId\application/vnd.wap.mms-message\Default
6. Change the Default data value to the GenId from step 4.
7. Make sure threaded sms & stock mms are not running and do a soft reset.

This seems to be the way others are getting this to work. It works perfectly for me when using SMS, but when I go to Add > Picture and send (resulting in an MMS) I get a "out of memory" error. And if I go into Menu > Options > Advanced, Manual is already selected. I tap Edit and there is an MMS url for sprintpcs.com. For those that have this working with MMS, what does your advanced settings look like? I would love to get this working. If we could get the steps locked down and clear it would be great.
Thanks.