Turn off both displays when in clamshell mode
You are confusing sleep and clamshell mode. Clamshell mode means using a laptop with an external display while keeping the lid closed.

Anyway, clamshell mode behaved differently in 10.6 and earlier:


Closing the lid put a laptop to sleep even when the laptop was connected to an external display
To use the laptop in clamshell mode, you had to press a key or click a mouse button after that
If you then opened the lid, the internal display didn't turn back on


You can restore the old behavior by changing the boot-args firmware variable:

sudo nvram boot-args=iog=0x0


You have to restart to apply changes. To undo it, run sudo nvram -d boot-args and restart.