Are there any consumer grade (i.e. not ridiculously expensive scientific equipment) digital cameras that use "grayscale" sensors instead of the prolific RGGB bayer filter? I figure that you'd get an extra stop or so of light if you weren't cutting out everything except the red/green/blue at each pixel. I've seen that some companies stock sensors by themselves but I'm not about to build my own camera from parts - though I wouldn't be averse to removing the bayer filter from a cheap enough camera.
Right now, no. Leica recently announced a monochrome version of the M9 digital rangefinder, but it is expensive (about $8000). Long ago, Kodak made a black and white version of its 760 DSLR (based around a Nikon F5 body), but that was also priced at around $8000, not aimed at the general consumer market. There have been persistant rumors that Ricoh will produce a B&W module for the GXR modular system, but nothing has materialized yet.
You're right about the advantages of a monochrome sensor, but there's two big reasons I think we're not likely to see this in the mass market:
First, modern sensors have great high-ISO performance and high resolution.
Second, starting with a color image allows very versatile black and white conversion in post processing. Traditionally, one might use a red or other colored filter with black and white film to get a specific look. If you start with a color image and do digital conversion later, you can choose the blend of wavelengths visually after the fact.
There's always demand for more extreme performance, so if it were just the first, there might still be a market, but the second means that it's going to be a niche market.