I've done this with compusa.
it's because of Best Buy's policy
at least when CompUSA has a big rebate, Best Buy will automatically match their price (i.e. you dont have to do anything, its already in the computer). so what do you do, you go to CompUSA and have them pricematch Best Buy's price. They call up Best Buy, and find out the price, and sell it to you for that price.
I got the WD 120GB (only 2mb buffer though) last August for $100 with rebate because of this (it was $300, there was a $100 comp usa rebate, so best buy was selling it for $200, so comp usa sold it to me for that price, and i got the $100 rebate a couple of months later)