source.android.com has build instructions for the stock distribution; you just need to figure out what the device name for your phone is (Nexus S == "crespo", I

source.android.com has build instructions for the stock distribution; you just need to figure out what the device name for your phone is (Nexus S == "crespo", IIRC) and specify that as the target to "lunch". You'll get a zip file you can then flash to the phone.