There are subtle differences between the ranges, but these only become significant in the hands of a skilled operator.
Nikon and Canon produce full frame DSLRs with a very comprehensive selection of lenses. Canon's lens range has a lot of overlaps which cover different market sectors, Nikon often has one consumer line and one professional line with nothing much in between. Pentax on the otherhand have rediscovered medium format with the 645D, this does limit your choice somewhat to the few medium format lenses for the 645 system out there. Nonetheless the large sensor and modern image processor are nice. Unfortunately, if you're spending £10,000 on a medium format system, you'd rather get a Hasselblad 503 with the CFV-39.
If you look at smaller sensored cameras, well the three are more or less equally matched. Pentax has the best backwards compatibility, however Canon can accept the most adapted lenses (pretty much any lens with an aperture ring), except older Canon FD lenses, this means you can mount the Nikkor-P 105mm f2.8 onto your Canon EOS, and get it to work (the Nikkor-P won't meter on low end Nikons). Nikon seems good for backwards compatibility, but you need to spend money on a D200, D300/S or better if you want metering on those older lenses. So if you have old lenses, get a Canon.
Canon's ergonomics are probably the best of the three, they seem to be designed with someone wearing thick winter gloves in mind, so it's deady easy to use.
It's really not much between them. I like Canon because I can mount OM, M42, Nikkors, Pentax, Carl Zeiss, all onto my camera. I also like Canon because you can get a full frame DSLR or £600 (a second hand 5D).
Your reasoning may be quite different. You may like Pentax's K-5 because of it's remarkable build quality and its low light performance. Or maybe Nikon because of it's Wireless CLS (aka AWL).