If you are using a Galaxy Nexus on Straight Talk with a AT&T provisioned SIM card and having issues with sending and receiving MMS, here are the APN settings that should work for you.
The Galaxy Nexus that I bought through Google Play had a Cingular APN already on it. Delete it. Or at least write down the settings just in case everything breaks on you. And then delete it.
Add a new APN with exactly these entries:
Name: straight talk (or whatever you want it to be)
Username: Not set
Password: Not set
Server: Not set
MMS proxy: 126.96.36.199
MMS port: 80
Authentication type: Not set
APN type: Not set
APN protocol: IPv4
APN enable/disable: APN enabled
That should be it.