Using my trusty Kill-A-Watt power meter, these are the measurements I got for the iMac:
Off - 2 Watts
Sleep - 2-4 Watts
Idle, screen off - 55 Watts
Idle, min brightness - 71 Watts
Idle, mid brightness - 96 Watts
Idle, max brightness - 157 Watts
With screen set to mid brightness:
Booting up - Up to 184 Watts
Transfer file to iMac over Ethernet - 125 Watts
Cinebench OpenGL - Up to 142 Watts
Cinebench multi-CPU - Up to 196 Watts
With screen set to max brightness, the Cinebench multi-CPU benchmark can bring the iMac's total power utilization to 257 Watts.
This means the screen power can vary by almost 90 Watts between the lowest and highest brightness settings, and for the CPU, going from idle to near full usage, total power utilization will increase by 100 Watts. TDP of the Lynnfield Core i7-860 CPU is only 95 Watts (not 100) at max usage, but that doesn't include the increased power needs of other parts of the computer such as memory, etc. when the computer is run at full tilt. The power utilization would likely increase even more with heavier usage of the hard drive and GPU.
Given these numbers, we shouldn't feel the need to completely power off our Core i5/i7 iMacs to save energy. Sleep is good enough, as the difference in power usage between sleep and off is negligible. However, if you leave your computer on 24/7 without sleep, the machine will still use about 55 Watts (when the screen is off). At say 14 hours a day in this mode, that's 0.77 kilowatt-hours. At 11¢ per kilowatt-hour for example, that works out to about 8.5¢ per day, or $30 per year.