I would like to offer this solution. I have a MacBook Pro with an external monitor, and, for a year and a half both displays slept when I closed the MBPS. This behavior stopped when I installed Lion. They would sleep when I removed the power cord, but what a pain that would be. The command "pmset -g assertions" gave no indication of which process ought be preventing sleep.
I contacted Apple support, and after some searching, the guy working with me found this thread, which was flagged as an Apple-approved solution:
https://discussions.apple.com/thread/3190417?start=165&tstart=0
This is the important part:
Correct AnswerRe: How do I tell Lion to disable the LCD screen even when the lid is open?
Aug 31, 2011 9:56 AM (in response to Bob_viking)
Here's the command to make your laptop behave like it did Pre-Lion (courtesy of my friend):
sudo nvram boot-args="iog=0x0"
Works perfectly for me. If it screws up your system, just zap the PRAM next boot (cmd-opt-p-r) and you'll be back to the default Lion state. Or if you can still get into terminal, this command will get you back to Lion's default state as well:
sudo nvram -d boot-args
Enjoy!
One snag was that I had to run it with the external monitor disabled, but it worked great!