The GP2Y0A21YK0F is a better sensor in that it works as close as 6cm. Mouser has them on back order for 3 months. I can not be sure but they may list soon-to-be-stocked items at a discounted price to get a few sales prior to getting the shipment. The GP2D12J0000F was cheap and back ordered, when they became a stocked item the price went from $8 to $10. It will be interesting to see if it happens again.
Try DigiKey--at least the Canadian site lists the GP2Y0A21YK as in stock. Still $10.71 CDN a pop though.
I seen an article on the web where a guy took the PCB off the sharp sensor and added a spacer to change the focal length. Shortened the range. I can find it if anyone is interested.
Oh yes--another reason I agree ultrasonic might not be great for a maze: unless you tuned things down quite a bit I'd think you'd get crazy reflection problems. There would be pings bouncing everywhere in such a confined space. It would probably be possible to deal with that but I don't know if it would be worth it.
Torben
[Edit: I know ultrasonic isn't really what you're looking at; I'm just spewing thoughts as they come.]