Here's some actual power consumption numbers for a fully-loaded 27" iMac (3.4GHz i7, 16GB RAM, 256GB SSD+2TB HDD, 6970M 2GB VRAM) *and* a 27" LED Cinema Display. An Apple Wired Keyboard is plugged in, an iPhone 4 is plugged in and charging, it's connected via 802.11n, and bluetooth is enabled (Magic Mouse or Razer Orochi).
iMac only, basic use, 50% brightness: 95W.
iMac and display, idle, 50% brightness: 120W.
iMac and display, basic use, 50% brightness: 145W.
iMac and display, compiling and light gaming, 50% brightness: 180W.
These measurements are consistent from both an APC Back-UPS RS700 and a P7 Kill-a-watt.
I haven't measured, but I assume heavy gaming is more around 210-220W for the full load (again, including a display here). This is still less than a Core2 Duo gaming PC with an nVidia 8800GT I built back in 2007, with a 23" HD Cinema Display hooked up under similar conditions (heavy gaming).