Okay, I have it all figured out, I think. I am going with the Sharp GP2D12 as suggested by BeanieBots, a 08M PICaxe and some random servo.
The code is all written and allows you to delay a flush for half an hour in case you have old plumbing (like I do) and need to use the toilet, then take a shower. There will be an indicator light, so you know if it is in 'delay mode'. It will also only activate if someone is in front of the toilet (between 5 in (~12 cm) and 2.5 ft (~76 cm)) for at least 5 seconds, so it will not be activated by walking past it.
I plan on installing the PICaxe and servo in an enclosure inside the tank, to make it easier to pull up on the flapper valve, instead of pushing down on the handle. I think I will install the delay mode button, indicator light and IR sensor in a candle that can be placed on top of the tank, so it doesn't look too conspicuous.
I will post the complete project with code, pictures and whatnot in the projects section pending my girlfriend's approval (she's scared I'm going to break the toilet or something) and project completion.