Go with a Nikon or Canon and you'll have more options and more convenience when shopping for new toys to mount on the body, like flashes and better lenses. Of course, you'll pay for that convenience by going Nikon or Canon, but in the long run it'll be worth it just because of the upgrade path that's available.
In today's age of interweb shopping I think this argument is BS. Go to B&H or Adorama or KEH and you can find anything by any manufacturer, new and used, and have it shipped to your door in 3 days with no sales tax. It just bugs me when people say you have to go with Canon or Nikon or life will be so difficult. By all means if Canon or Nikon has what you want, then go with it, but don't be scared of Sony or Pentax either if they have what you want.
Once again, tell me what lens, or flash system an amateur could want, and afford, that you can't get with Sony or Pentax? Aside from the reeaaalllly long, fast $6k+ telephoto (although you can still find some Minolta 600mm f4's around) and some T&S lenses they've got it covered as well as Canon or Nikon.
I started out with a Minolta Maxxum 9000 (first ever autofocus SLR) and have grown with Minolta/Sony ever since. I liked Minolta back then because I felt they had the best ergonomics, and my buddy had it too, so we could swap lenses. (and I still think the Maxxum 7 is the best film camera ever made) Besides a few scary months after Minolta sold out and before Sony made a serious DSLR, everything has been great. Sony/Minolta has always had what I want (most recently a full frame 24 MP body for less then $2000), in body image stabilization, and lots of great old Minolta (and new Carl Zeiss) glass. That said I could be just as happy with a Nikon system, the aforementioned buddy now shoots Nikon, as he got tired of waiting for a Minolta digital before the 7D.
Recently Sony (and to a lesser degree Nikon) have been eating away at Canon's SLR majority market share, I think this is a great thing for users of all brands, more competition = more innovation and lower prices for everybody.
BUT Sony doesn't have a 10fps camera,
or a great low light camera
or video,
or a 800mm f5.6 lens, (someday they might) So if you want those features go with Canon or Nikon.
It's all about what you want/need, heck if you wanna do street photography get an old Leica M rangefinder, don't know why but they seem to be very popular among cityscape/street photographers. If you wanna do stunning landscapes that can fill a wall get an old Pentax 6x7 and some Velvia/Provia film, I love mine. If you want to do
REALLY stunning landscapes that can fill a billboard get a 4x5 view camera. If you want a really small camera that will fit in your pocket, and still have interchangeable lenses get a Panasonic G1.
Please, I'm
NOT a brand fan boy. I just don't think you should feel like you
have to buy a Canon or Nikon, gear
IS important, but the person behind it is much more so. Research all the different systems and get whatever suits your needs best. Plus as a beginner with only one body and maybe 2 lenses, it's not a huge loss to switch systems once you learn more about photography and discover what you want/need in a system. It's once your really into it with 2-3 bodies and $10k worth of glass that it is a huge deal.