Android early boot console over usb
This site describes how to get access to the serial console on the Nexus S, which is really the only way of debugging boot problems. It's not for the faint-hearted, though: you need a UART-to-USB module, with resistors connected across certain pins, all connected to your phone. If you screw it up you will blow up the phone.

The same information's also mirrored here, in case the first site should disappear.

BTW, if you'd tried asking this on Stack Overflow instead of here, you'd have found this duplicate question which has the same link I just found :-)