Problem: Macbook video firmware may become disrupted on a machine that has MS Windows installed via Boot Camp. This will present as a failure to boot, or a failure to boot completely.
Workaround: When machine fails to boot, the startup sound will sound, but the screen will not display anything. (External monitors will also not display anything.) Press the power button until the machine starts to restart, then press and hold Control-Option-P-R simultaneously to reset the PRAM. Do this three times through startup.
Long-term workaround: Always start the machine while holding down the Option key to choose which boot directory to use.
[robg adds: I haven't experienced this issue myself, but I haven't used the Boot Camp Windows install on my MacBook all the often.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060731224456989