If you turn on a MacBook when it’s completely powered off (not sleeping), and instead of starting up, the screen remains dark and the sleep light is blinking, the MacBook’s RAM might not be installed properly or might have worked loose. I found this out when re-seating the RAM of a friend’s MacBook to try and fix a problem. I thought I had pushed the RAM modules far enough into their slots, but after the MacBook failed to start up I took another look and found out that you really do have to push firmly and carefully past some initial resistance until the RAM goes in all the way. You may have to push harder than you think is normal, but seriously, you have to push pretty hard.
If your MacBook has metal levers near the RAM, don’t use the levers to do this. The levers are only for popping out the RAM, not inserting it.
I wasn’t able to find a tech note at the Apple site about this, and that’s why this entry exists. In case it helps someone.
(Update: It seems like this applies to newer Macs such as the unibody aluminum MacBook Pro line and the iMac, although I haven’t tried it myself. However, it won’t work with a MacBook Air or a MacBook Pro with Retina Display, because on those models the RAM is permanently soldered to the motherboard.)