I just detailed my engine bay:
http://www.camaro5.com/forums/showthread.php?t=390747
All you need is some all purpose cleaner and a bristle brush of some sort. As you can see in my thread, I covered the air intake and alternator.
Use a low pressure on the hose and hose down the whole bay, apply APC all over, agitate with the brush, allow it to dwell and then hose off and dry. You can apply dressing if you wish to the black plastic, I decided not to.