To second DoubleLori's comment, there are several computers that I know of that allow the user to just enter wheel size to calibrate the computer, but these manufacturers also caution that this is a rough calibration at best, and they recommend that if you want more accurate readings, you should use a more accurate measurement.

You can get this accurate measurement either by measuring the distance your wheels travel in one revolution with you sitting on the saddle (you need an assistant and a tape measure for this one), or, as DoubleLori suggested, you can ride a measured distance and adjust accordingly. If you have any rail trails in your area, you might find that they are marked off in miles.

Lots of sellers of biking accessories are now marking down their 2001 stuff to make room for 2002 models, so it might be a good time to look for last year's computers. They shouldn't change that much from year to year, and you could get yourself a bargain, which is especially good if you decide to look for a wireless one--they're generally more expensive than the wired kind.

Let us know what you end up with.
--J**