I have a 3g iphone with tmobile. I have screwed it up and need help BAD! I have been trying to fix for about 10 hours now. Here is what happened:
I was running 2.2 firmware (jailbroken and unlocked) and I upgraded to 3.1 via itunes. I then found out that there wasn't a jailbreak available for 3.1 yet. I then restore it back to factory setting (I guess) via the itunes option under upgrade. Now it is not unlocked so my sim isn't working. All that is showing is the screen that says "no sim card installed" and a picture of a cord pointing to itunes. I am not sure what firmware that is installed because this is the only screen that is available. I also have no idea how to jailbreak it from this screen without know what the firmware version is on it currently. Can anyone PLEASE help me fix this phone! Thank you in advance!

Well, since you have a iPhone 3G (not 3G[S]) you actually are able to jailbreak 3.1 right now using PwnageTool 3.1 from here.
Since you upgraded to Apple's official 3.1 software beforehand, your baseband has been updated. Right now there is NO solution whatsoever to unlock the new baseband.
No one knows if there will be an unlock in the future sometime, so you are out of luck using your T-Mobile sim.
Sorry, nobody can help you unlocking your phone right now.
Check the link above frequently for new updates on the matter.