I go against what all the rest say. I cover my monitor with a black cloth when profiling. The puck is reading from the screen and I don't want any ambient light leaking in from the sides. All of the above advice is great for viewing, but I have been told by a consultant that you want as pure a reading as you can get from the screen.
There is a large amount of truth to this. Your goal is to measure the output of your display, not the ambient lighting. Different measurement pucks and display technologies are affected differently by ambient light. what you want to avoid is the sensor picking up light scattered from the sides. If the sensor is reading light that comes from the room rather than the display, you will not have any accuracy in the screen's shadow response. We have characterized a number of measurement pucks for an upcoming report on sensor accuracy and found large (>15 DE) differences in measured black levels when comparing low light vs. typical office lighting for the ambient. The worst pucks in this regard were the i1-Pro and Spyder 3, the best the i1-Display.
Draping a cloth over the monitor works in a brightly lit environment. If you have control of the light switch, however, this is a better approach. Some sensors are prone to overheating and skewing the calibration levels. This is compounded if you use profiling software that takes a long time to do its thing.
As far as viewing environments go, the comments by Pat, Andrew, and Walter are spot-on. Avoid having light shining directly on your screen. A simple hood makes a major difference.
The overall lighting level should not be overly bright, but today's LCD screens allow more flexibility than past models. Decent quality graphics displays offer the best overall color gamut and output smoothness when run at a luminance of at least 140 cd/m2. If your editing environment is still tuned to the 80 cd/m2 luminance of yesteryear's CRT displays, this will seem blindingly bright. If so, come on out of the cave!
If your ambient light is too bright you will not see the full black range your display shows. Profiling the screen with the light on will not help; the software tries to compensate for light that does not originate from the display. The result is a profile with poor shadow definition and, usually, a color cast as well.