I didn't think it would do anything, but...wait a second...just sent and recieved MMS successfully as I was typing this.
In GoSMS, go to Settings > MMS settings, check Fill APN configuration manually, set MMSC to
http://mmsc.vmobl.com:8088/mms? MMS PROXY = 205.239.233.136 and MMS PORT to 81
edit -- I'm sure there is some system file this can be changed in to make MMS work with the native messaging app, but I thought someone in this thread said there was a problem with the stock app. I don't know anything, clearly, but hopefully it helps for somebody.