Well, here are the reasons I went with the the Canon T1i over a comparable Nikon...
1. It has the lenses I want. Someday in the distant future when I actually have money, I want to be able to buy some f/2.8 super-telephoto lenses, because I really want to get into bird photography. Canon is renowned for their 300+ mm L-series glass, and as far as I know none of the other brands offer anything that can truely compete with Canon in this segment of photography.
2. At the other extreme, only Canon has a 5:1 magnification macro lens. This is something I want to master someday, since I am greatly enjoying photographing arthropods with my 100 mm macro. Sure, with other brands you get a lens with lesser magnification and then add some extension tubes and whatnot to them, but that seems like an unnecessary hassle and the image quality is bound to suffer somewhat. I'd rather be able to just buy a single, high-quality lens dedicated to extreme magnification.
3. Nikon lenses screw in the wrong way (counter-clockwise). Sorry Nikon fans, but this was just incredibly annoying when I was playing around with various Nikons and I just couldn't get used to it. Oftentimes I need to change lenses quickly, and that sort of thing would piss me off pretty quickly when I would need to remember to screw the lenses off in the
opposite direction than what is second-nature to me.
4. The Canon T1i is a smaller camera. Since I have small hands, the Canon was more comfortable for me to use than the bigger Nikons. It is also very lightweight.
5. Canon, from what I've heard from photographers, camera store employees, and people on here, seems to have better, faster customer service and repairs. In fact, I've seen many people complain that Nikon's customer service (at least when it comes to entry-level cameras) is just plain shoddy sometimes, and more expensive as well. I haven't had to get any repairs on my T1i, but last year when I sent in my PowerShot G9 for a broken lens pin (as a result of me dropping the camera one too many times), the repair took just a week and a half and cost me only about $170. I don't know if it's different for DSLRs, but I read on here that Nikon takes two weeks just to process the repair order and there is a minimum charge of $250 just to look at the camera. :shock:
Anyway, I would say for most forms of general photography, it doesn't really matter which camera you get, because both companies offer very similar products in that range for a similar price. For both the T1i and the D5000, the image quality is going to depend on your photography skills and the lenses you have much, much more than the camera itself... The camera can't make you take good pictures, only you can. When it comes to the more specialized, niche markets mentioned above, however, I do feel that Canon has the edge.
Whatever camera you do get, I think you should skip the kit lens for both brands though. When I was trying out both the Canons and the Nikons in stores, I found the lack of sharpness and definition from the kit lenses very frustrating indeed. But I think that was because I was used to the crisp, colorful images from my PowerShot G9, which sort of spoiled me. Many people think the kit lenses are fine, at least for a while.
EDIT: Here are some images I've taken with my T1i:
