Fun and inventive!
First off, I will say it's nice having a chance to actually play this without it causing my browser to almost completely freeze up. ;-) So thank you for that.
As for the game itself, it's definitely got an interesting premise, and the level-flipping mechanic is used quite well to make for some good puzzles.
I will say, though, the lack of instructions is a sore spot; having to run my fingers along the keyboard to see if I can get my character to jump just to start the game isn't something you want the user to do. So I'd put in something on the title screen, or a link to another window explaining them.