Well done.
Very well done, but the visual slowdown that other people have been talking about hindered me from playing very far.
It is not my computer, I'm currently running a 2GHZ Core Duo with 2gb Ram and Nvidia 8800GS.
I've noticed that the game only slows down in certain parts of the level. It would run fine in some parts, but if I went back a little ways to where it was slow it would slow right back down again. Maybe there are too many objects being rendered in that area of the level? That or maybe you're having collision overlaps or any number of other things. Just wanted to hopefully provide some technical insight into what your problem may be.
Otherwise, well made game.