I did use a color vision spyder 2 to calibrate my projector. It works with some limitations:
1) My projector is pretty OK when set to RGB
2) The spectral properties on the lamp of my projector are not optimal. It cannot yield any pure red. The calibrator can do very little about that.
Another issues is that there are only few programs that actually use color management on the Windows platform, many bypass both calibration and profiles.
Acrobat reader and Quicktime seem to work, though.
That's my experience, your mileage may vary...
Colorvision's Spyder 3 Elite claims to be able to calibrate a projector. Have you used it for that purpose? If so I'd like to hear about your experience both pro and con.