It's for THE BIG WHITE LENSES. No one gets out of your way at the zoo or on a trail if you're carrying around one of those wimpy little black nikon tubes. But oh baby, you pull out that big white baby 100-400, and they scamper away to a safe distance and peer around trees at you, muttering things like "Ohhh- now that's a REAL camera," or "Look honey, he's a professional..."
Actually, I went Canon because I prefer their picture style, and the comparable gear is cheaper than Nikon- that extra 30% cost of Nikon gear's a killer. also, when I went into digital slrs (actually, when I got my wife one, which then subsequently led to my addiciton), Nikon had nothing even remotely comarable to Canon's line up. They've only come out with something comparable to the 5D this year, and the 5D is what, three years old? BUT, the new stuff from Nikon- the D700 and the D3, are pretty sexy. Still, if you are looking at buying a whole new system, lenses and all, and you want professional results on a budget, Nikon's really got nothing to compare to the 450D, the 40D or a 5D, all of which are something like the value of a lens cheaper than Nikon's rival, and in some casess (450D, 40D), I reckon superior.
But at the end of the day they all take great pictures.