ctriplett...is your phone rooted? I'm asking because I just dealt with this problem myself. After rooting my phone, I downloaded GO SMS to use as my default message app. I DELETED my stock messaging. Didn't realize that I wasn't receiving mms messages until my husband tried to send one from his new phone. Turns out that since I removed the stock messaging, *nothing* was handling my mms messages. I had to go into GO SMS >Menu > Settings >Advanced > Important Tips and check the box that states "Stock SMS uninstalled." Then I was finally able to receive mms messages again.
Hope that helps or at least gives you a place to start. :-D