Thought I'd weigh in on the discussion.
Canon and Nikon each have their pros and cons. I've been a Nikon shooter since I was 16 and bought my first "real" camera, an FG-20. My son is now 16 and we got him a D3100 for Christmas.
If I were starting from scratch, I may very well go with Canon for a few reasons. If you're looking to build a system, it seems there is more Canon gear available second-hand, at least in my area. I also like that their ettl system works with their lower-end stuff, while Nikon restricts CLS to prosumer and up models.
(Having said that, I'm teaching my kid to properly use manual settings with a collection of vintage flashes --- no cheating. Just got some replacement metal bases for my 285s through ebay that I'll be soldering on this weekend).
Yet I still find Nikon gear more comfortable to use. Ergonomically, I think it's better and I find it more intuitive.
What it really comes down to is glass.
Bodies come and go, and should be considered on a replacement cycle more on the level of computers than cameras these days. I shot my F3 for a good 10 years and it still works like a charm (though looks like hell).
You can purchase a top-of-line DSLR today for several thousands (hello F4s) and it will be a paperweight in 10 years. I have a D1s in my bag to prove that.
However, the lens you buy today will still be solid performers a decade from now, provided you treat them well.
Agree with Brad that Sony is the dark horse in all this. I have no experience with the system but it seems competitively priced and feature rich. A solid electronics company with which my only complaint is it lean toward proprietary technology. Then again, I've used Apple pretty much exclusively since 1987 so I'm kinda talking out of my ass on that one.
Like Brad, I'm sticking with Nikon, because I've already build a system (that's totally beat up but still functional) but I'd be hard-pressed to pick Nikon over Canon over Sony if I was starting from scratch. They all make solid products, though I've found Sony's customer service somewhat better than Canon's or Nikon's.
Canon has the sand in mechanism and Nikon had the oil on the sensors of the D700. QC is an issue all around.
I'd say choose the lenses you like, the ergonomics that work with you and build from there.
(PS, look back one post for my Johnny Winter pix
)