I've apparently been living under a rock, because this was new to me. Via Hammet, there is a .Net equivalent to WATIR called Watin at
http://watin.sourceforge.net/. They've even got a Fitnesse fixture built for it as well. I was pretty happy with Selenium when I used it last year and it has the key advantage of being cross browser, but I'll definitely give Watin a spin next time I do web development.
Of course, I betcha Bret would tell you that the big advantage to WATIR is the Ruby language.