It's quite possible that if you bring a JB iPhone into the store, at least if it's an Apple store, they'll know it. I read an article a while back about how when they plug iPhones into their computers, it automatically searches the iPhone for the JB processes and files on your iPhone.
Your best bet is to probably install a custom 3.1 firmware with SSH enable. You can then SSH into the iPhone and move the Cydia.app folder somewhere else, which will remove the icon. At least this way, they won't see Cydia installed and you don't have to worry about SBSettings popping up or something like that (stuff that hides icons). After you come back home, you could just SSH back in and move the folder back, or just restore again to the custom firmware. Even without Cydia there, it's still JB, so it'll continue to accept a custom firmware.
You do not need to unlock your iPhone before going in there, you just need to make sure your baseband isn't upgraded. If you install a custom firmware, your unlock option is preserved.
HOWEVER -- why don't you post your MMS issues here and see if we can help. You might not even have to go into the store. Chances are, they are going to suggest a restore, which will remove your unlock option.
(I hope all this makes sense, I'm still workin' on my morning coffee)