Why can't I run my MacBook Pro in clamshell mode? I'm following these instructions to use my mid-2009 MacBook Pro in clamshell mode with a Bluetooth keyboard and trackpad. The keyboard and trackpad are paired with the computer and I've got the external display hooked up, but the computer is going to sleep when I shut the lid. I have Allow Bluetooth devices to wake this computer checked in System Preferences, so I can hit some keys to wake the computer while it's closed. It wakes up and shows me the lock screen, where I start typing my password... only to have the computer go right back to sleep after a second or two. 

Why won't it stay awake?

Clamshell mode behaved properly when I was using a USB keyboard. 

I'm currently using NoSleep to work around this problem, but I'd like to know the root cause.
The only time I have seen this model fail to work reliably is when there was a problem with the AC power (and it then was continually, but momentarily on battery and sleep is the correct answer) or when the lid isn't securely closed (a drop has bent things inside, or the lid doesn't close firmly, or the stand allows the lid to wiggle open/closed a touch.

Have you poked around a bit to see if any of the things can be isolated (lid movement, power supply, connecting a USB device just to avoid a bluetooth drop from making it think it should sleep now).

You don't need these changes permanently, but just to isolate why it might work sometimes and not another. If it fails all the time, you might need it serviced since Lion has worked out most of the sleep/wake screen issues associated with clamshell mode. (and there sure were a lot on 10.6 when we didn't know the VRAM was being allocated, but not reallocated on lid opening events as described in the support article you mentioned)