Really, don't do this. The dev team is coming out with a relock tool any second now.
Apple has your unlocked iPhone in their crosshairs. What do you do? This method in no way guarantees that your iPhone will survive the update after being unlocked. If you're risk adverse, you may want to wait a day or two for these instructions to get debugged. And be aware there may be a lock/unlock limit set by hardware.
Important note: people are experiencing munged IMEI's. "Delayed Green" says that he saw this "Incorrect SIM" error in the early unlock days and that he knows of no way to fix the problem. The IMEI errors seems to be unrelated to unlock method--I'm hearing from people who used pretty much each of the unlock scenarios. Quite a few people have simply re-unlocked and are using their phones without problem but can't re-lock, that is reverse the unlock. Can you put the genie back into the bottle? Reliably? Apparently not yet. And the methods previously published just aren't working properly. If you haven't tried relocking yet, do yourself a favor and just wait until we figure out why some relocks are going wrong. Update; iPhone unlocking team promises relocking tool within a week.
TUAW Reader Peter writes: "The reason why we're getting the Invalid SIM error seems to be because running bbupdater with the .fls and .eep files somehow causes the phone's IMEI number become 004999010640000. Running AnySIM or using any of the other unlock methods seems to "program" the iPhone again with the correct IMEI number. There must be another step required to reflash the baseband and get the correct IMEI number back. "