Greg Gable- Sorry, a little late since you've gotten your credenza I assume it's in there now, but the fans may actually be speeding up with the console on the floor, as I've heard on previous generation consoles that it was generally not recommended to have them on carpeting when playing (I assume your floor is carpeted?). With the console in direct contact with the carpet underneath I think it tends to keep the underside more insulated and warmer than it would be on a shelf, hence a better chance of overheating.
I do have my 360 behind a glass door, but I have 2 12cm pc fans connected to the receiver that power on with it. They sit on either side of the 360 to move the air out the back of the shelf.