I imagine that if you used networking cable that you could get the distance up to some tens of feet as it is designed to have very low capacitance. The alternative that I'm sure you've though of is to have two NXTs connected with RS-485 and have one take readings and pass them on to the master. The longest distances you might
be able to achieve is about 15 meters. This is probably not very easily achievable and will depend on clock speed, cable, the type of slave you have on the other end, solar flares and UFOs.
You'd probably have better luck using a twisted pair (Cat5?) for the data and clock. You could try shielded wire but unless it's made for hi speed communication it would probably have to much capacitance. You could try taking the outer layer of shielding off of a shielded wire and sticking it over some Cat5 cable. It might still increase the capacitance too much but its worth a try.
-EDIT- Check out this link, it has some good suggestions [LINK