"Tech Notes: Frame Rates, Clipping Planes and Horizons, Oh My"
As well as forum posters on a couple of other gaming sites, Butch mentioned in our forum that at times the frame rate of Halo did drop noticeably-- perhaps as low as 10 fps, in his estimation.
However, in addition to noting that Halo is still at least six months from release, Butch mentioned a few other things that indicate just how powerful this engine-- and the hardware it's running on-- really are.