Well, I actually went out and bought such a detector. The instructions pretty much say to place tham anywhere you want- they even discuss setting them atop a table.
There is quite a list of places NOT to place them....primarily places where either the environment will damage them, or cause false alarms. "Not within 5 ft. of any cooking appliance" for example.
I also found an interesting note:
"seven years after the initial power-up, the unit will (signal) it is time to replace the unit. The unit will NOT detect CO inthis condition.
