I know this is a "is it plugged in" kind of response, but have you installed BootCamp v1.3 drivers? If not, the keyboard backlight won't function without a 3rd party app.
first try installing the new drivers from the Mac Support website, or try to reinstall the drivers on your original Mac OS X install disc 1 CD.
If that doesn't work, Dmitri has developed a beta 3.0 software that enables the keyboard backlight to work, and you would also be able to adjust the sensitivity too, which i thought was definitely a great feature: