I was able to get MMS working by using the Wireless Manager app from the Marketplace.
I can now send picture attachments with my text messages.
I started Wireless Manager and unchecked "Automatic APN".
Set "Active APN" to AT&T.
Press "Edit APN"
Enter these settings from the Straight Talk APN Settings Card:
Proxy Address: 184.108.40.206
Proxy Port: 80
Press "Done" (Checkmark)
I hope this helps.