I haven't played with iPhone firmware 1.2/2.0 yet, but I'm told that some developers have encountered what is becoming known as the PSOD: a pink-colored start screen that apparently replaces the "connect to iTunes" graphic you'd normally see before activation.
iPhone hacker AlJaMa wrote in with his how-to method for downgrading a 2.0 iPhone back to 1.1.4. According to AlJaMa, you can put your iPhone into restore mode, do a 1.1.4 restore (use option-Restore to pick the 1.1.4 ipsw package) and be no worse off than getting the Error 1015 we've all come to know and love. iLiberty or INdependence will kick you out of the 1015/recovery mode. The restored phone will continue to work as a phone despite the updated baseband.
AlJaMa warns that you may encounter error -2002. In such a case, he suggests you try restoring to other firmware versions until you get back to the 1015 error.