I was trying to unlock my 3GS (16GB, 5.01, 05.15.05) so upgraded to 06.15.00 (iPad baseband). I didn't really care about GPS functionality, and I knew the risks involved.

Long story short, I messed up when trying to jailbreak using 5.0 ipsw for 5.01 firmware semi-tethered (i heard it worked...??) and now I'm back on the itunes activation screen. I have the original sim in the phone, and it's saying 'there is no sim card installed in the iphone you are trying to activate'. I have sn0wbreeze, and a custom ipsw that hacktivates it that I made using it. Should I jailbreak and then shift>restore>custom ipsw? HELP

The unlock on 5.0.1 requires a patched 5.0.0 commcenter, search through the forums for it. I'm not sure why sn0wbreeze didn't hacktivate for you (sn0wbreeze has proven to be buggy and unreliable in the past, so we try to recommend PwnageTool or redsn0w directly if possible). You may want to use redsn0w to make a custom firmware and restore to the phone in pwned DFU mode?