That was unexpected, and I didn't believe it at first. So I ran my own test.
10,000 64x64 sprites, unlimited framerate: 300fps
10,000 64x64 tiled bg's, unlimited framerate: 230fps
P4 1.3 Ghz, GeForce 6200
I'm stymied. All I've ever heard is that tiles are lighter than sprites. I'm also very curious why mine ran better than BenH's, because he has a better computer and a better graphics card.
Oh, and Mipey... if my crappy computer can handle 10,000 tiles in a layout that well, then you pretty much don't have to worry about your game.