I tried to do the ad-matching thing at Walmart last year but found that they didn't carry a lot of the items I was looking for.... mostly video games. (This is a regular Walmart, not a supercenter) Still managed to admatch a few CDs and DVDs, but couldn't avoid going to Best Buy as I had hoped.
I also got turned down at Circuit City trying to admatch a DVD because they said the item was "already on sale" there. My argument that is was on a better sale at Toys r Us didn't seem to matter to them.