Here is a simple procedure to reset the service countdown clock that actually worked. I found it after searching some forums such as Audi World. Many of the forums listed it with not quite proper English so it took me a few minutes of playing around to actually get it to work on my 2000 Audi A4 1.9 TDi.
Here it is as clear as I can make it:
0. Display reads "Service!" when you first start the car.
1. With the ignition key in the off position, push and hold the right button under the speedometer. This is the button you use to reset the trip odometer also.
2. Turn the ignition key to the on position, but don't start the engine. Keep holding down the right button while you turn the key.
3. Push the left button under the tachometer. Your service light should reset to say something like "Service in 8100 miles."
Some geniuses are selling these instructions on ebay for a couple of pounds. I say best of luck to them! Maybe I can make a few bucks that way
Now, if i can only find how to reset my clock!!!