Turn off both displays when in clamshell mode

It may be easier to use the "Sleep" option in the  menu (upper, right). It sounds like the "clamshell mode" you're talking about is actually "Sleep mode" to the computer. I believe that this is the most effective way to get things into "sleep mode."

Other options:

• Setup a "hot corner" that puts everything to sleep.
• Use keyboard commands for sleep. (Command + Option + Eject)

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.

I had this issue too, and came up with this script that monitors your lid state and puts the machine to sleep when lid is closed: https://github.com/pirj/noclamshell