Well my internet access ($10 Unlimited Web on EM+ 500min plan) is through internet2.voicestream.com. MMS is per wiki using wap.voicestream.com w/ proxy settings.

If I turn off the proxy on wap, I get an upgrade notice. MMS won't go through on internet2 w/ proxy.

The MMS don't show up in messaging, but it does show data used at about the right times (on my recent usage online). I expect they will charge me, but it would be cool to get "free" MMS through my data.