You know thats interesting hear. I've never had a dealer giving me a discount on warranties. But then again that was from Dodge on 3 occasions on the cars I've owned in the past. Normally if I buy the car with the warranty, the car will get marked down a little, but the warranty is pure profit.
That rule of thumb goes for lots of other things too like CompUSA. Their "Technology Assurance Program" or TAP is their extended warranty. It has anywhere from 75%-85% profit margin. That makes up for the low markups on the computers that only have 2%-10% markup.