If, like me, you incorrectly typed in your SIM card PIN number three times, the Nexus One locked itself down and presented a screen saying that the SIM card was locked and that a PUK code was required ("Read User Guide or contact Customer Care").
Anyway a quick search of the Nexus One user guide for PUK, and the answer was revealed (link below for info).
1) Contact your mobile provider (from another phone) and ask for the PUK code. After a security check they will give you an 8-digit number.
2) On the Nexus One tap "Emergency calls only" and you will be shown the usual dialer (don't worry, it won't call any emergency service, just show you the dialer!).
3) Think of a new 4-digit PIN number.
4) Type this code into the Nexus One's dialer:
**05*<PUK number>*<New PIN number>*<New PIN number again>#
For example:
I received the PUK code 62862993 from my mobile provider and I want my PIN to be 1234:
**05*62862993*1234*1234#
Get it right and your phone & SIM card will be unlocked and return to normal operation (I was too scared to find out what happens if you get it wrong...!).
Reference link:
Nexus One user's guide - Nexus One Help
..scroll down to "Turning the phone on and off" section.
Have fun!
Nick