And for those recommending Nikon, if you're shooting sports - then Canon it is. If you ever look along the sidelines of a sporting event, you'll see the distinctive white of the L-series large telephotos on almost every tripod. They'll usually be attached to a EOS 1D, a pro camera designed for outdoor sports photography - its weather sealed and has a high frame rate with a 1.3x crop sensor.
1. If you look on the desktops of home users and companies, you'll see PCs. Does that make Apple a bad choice?
2. Canon offered heavy discounts and loaner equipment to media companies and sports shooters for a lot of years.
3. Until the D3, Nikon hadn't updated its professional sports body since the 4MP D2h, leaving sports shooters with little choice.
4. Please explain how the AF system in a 1D translates to the AF system in a 5dmkII or 50D.
5. Please explain how the frame rate of the 1D translates to the 5DmkII and 50D.
6. Please explain how the weather sealing in a 1D translates into a...
7. Please explain the exponential growth rate of D3 and D700 bodies at sporting events given the price of switching systems.
While the marketing folks at Nikon and Canon indeed hope that people will think that seeing their cameras in a certain place or for a certain use will translate to people who have no intention of purchasing their high-end models buying their lower-end products the truth is that such comparisons ultimate need a lot more analysis than "I saw a bunch of that company's gear at a (pick an event.)"
How does it translate? For instance, if Canon weren't spending hundreds of thousands on sending people and loaner equipment to sporting events, and Nikon weren't now competing to do the same, perhaps Canon could have afforded to put a more accurate AF sensor in the 5DmkII, and Nikon could have done more work on video AF in the D90.
The 1DmkIII is a great camera, the 50D is a great camera, but to equate the two is like saying that because the Nikon D3x is measured as the highest image quality camera this side of $22,000 the D40 is going to produce great ultra-large prints.
When you're shooting for money, small edges give you an advantage over the next person- the D3's edge in AF accuracy has a fair number of sports shooters switching. It's edge in ISO has lots of wedding photographers switching- but sometimes those switching are switching more for perception or to make up for a lack of skill than for a slight technical advantage- and without knowing a lot more, it's rather silly to try to apply a blanket reason to a swath of shooters with a swath of skills and motivations.
For instance, is a wedding shooter switching to a high-ISO body because of poorly lit venues, or because they couldn't light a cigar with a bonfire in the middle of an active volcano, or both? Are the ones who switched to Canon from Nikon now switching back because of performance, preference or stupidity? Are the ones switching from Canon to Nikon for the first time motivated by single-body quirks, hullabaloo from Internet fora, or something different? Are the switch numbers as large as they seem, or is it that switchers are more vocal? Are they all going to switch back when the next Canon comes out? How many people are buying one or two additional bodies and a couple of lenses for a particular purpose? How many people are spending large sums to get the "best quality" then throwing poor-quality filters with bad color casts on them and shooting above base ISO to give them a better chance of getting focus? Are they getting results that are as good as if they'd just gotten adequate gear? Do some shooters buy the high-end bodies just so Uncle Joe at the wedding won't make them look outclassed in front of the other guests?
I was actually at a rather large US media company (Gannett) when Canon came courting the photo department of our largest publication (USA Today)- when you're shooting for newsprint almost anyone can give you acceptable IQ, that's not the major deciding factor in which vendor to choose, and when your staff is already shooting old equipment (find me an industry that hangs on to old crap longer than the newspaper industry and I'll be surprised) anything newer is going to do better.
If your sum criterion is "A bunch of companies bought from that vendor, so you should do it for your use too" then you'd be running a Dell with XP on it while Microsoft keeps trying to get you to "upgrade" to a new OS.
If the OP is interested in Canon, then they should get a 40D, it'll do the job well and the extra money can go into glass.
