Consider this Sanus if you want to mount it on the wall:
It's good for up to 80 pounds (the new 27" iMac is 30 pounds). It extends 3 1/2 to 20 inches and is very sturdy.
I installed a 32 inch LCD HDTV on it and I'm very pleased with it.
My local Best Buy has a sample setup on display.
|AF4_2483 Surfing the Serengeti by DaveInHouston|
from Hot Air Balloon view
|Peregrine Falcon by Psychic1|
from Best Wildlife Photo of the Week - 4