I have used all three brands - Olympus, Nikon, and now Canon. ALL THREE take great pictures. A body is a body, you may want to pay more attention to the LENS that you buy. And you may have missed the Pentax K10D (cheap and very good) - EISA European Camera of the Year 2007-2008 plus Camera of the Year award by many foreign magazines - wonder why?

Olympus

1. Light, small (except their pro model) - body and lens
2. Being four/thirds system, many will appreciate their 2x crop factor (100mm becomes 200mm)
3. Lenses are generally cheaper than Canon/Nikon
4. Olympus is a very innovative company, as seen by their past photographic achievements (eg. first to introduce LiveView)
5. Better sensor cleaning system compared to Canon/Nikon

but.....

1. Its harder to buy/sell used Olympus bodies/lenses over here in Malaysia. People lean too much towards Canon or Nikon, hence the availability of many used Canon/Nikon items.
2. The 2x crop factor is bad for fans of wide angle shots

Canon

1. Generally, superior ISO performance (until Nikon introduced their D300/D3) - better for action shots (the 40D's 6.5 frames per second beats everybody else in its class)
2. Basic kit lens not optically great, but hey, Canon has those L lenses!
3. Cheapest fullframe camera (the 5D) in cameraland! Only company to have a few fullframe cameras in their line-up (again until Nikon introduced the D3)
4. Once more - some people switch to Canon just so they can buy a L lens!

but.....

1. Canon now has serious competition in the ISO game
2. L lenses are expensive

Nikon

1. Many prefer the colors produced by Nikon cameras compared to Canon (this is of course debatable, Canons tend to produce colors more true to the actual scene, that is sometimes interpreted as being a little dull or flat)
2. Lenses are slightly cheaper than Nikon (across the range)
3. Nikon now has the very impressive D300 and D3 - but don't forget the D300 body is almost rm2000 more than a 40d body, and it is NOT SUPERIOR in every way to the 40D, as mistakenly pointed out by someone earlier
4. Nikon has the very versatile 18-200 VR lens, a lens that Canon does not have (or rather, does not want to make)
5. NASA uses Nikons on all their space shuttles (they recenty purchased 76 Nikon D2Xs)!

but.....

1. D300 (and the D3) are expensive (the Canon 40D is selling cheaper now than when the 30D was introduced 18 months ago)