given everything you've said here, i would not recommend getting the cheapest kit. i did that last summer when i bought my d40. it was my first digital, and after two months i'd outgrown it, and i can imagine the same happening to you. as i used it more and more i found it lacking a lot of advanced features i really felt like i needed to get the shots i wanted. there was no mirror lock up, no bracketing, no built in af motor, no hi iso capability, etc. likewise i'd steer clear of the the rebel x series (xt, xti, xsi) for those reasons and more. at least among the lower end bodies, i think the nikons are much better built.
if you really loved the d90, i would HIGHLY recommend it. i'm a nikon guy, so i might be biased. but it's got enough features to keep you busy learning for a while, the sensor is pretty much the same one that's in the higher end d300 and has nearly the same or better high iso performance, and, gimmick or not, it'll do 720p hd video with all those beautiful nikkor lenses. i personally wouldn't get the 18-105mm kit lens that comes with it. if you can find a kit with the 18-70mm, i think it's a much better lens. or if you want to go old school, get a body only and pair it with the sigma 30mm f/1.4 (the equiv of the "nifty fifty" on the smaller digital sensors, and nikon doesn't make an af 30, yet).
i'd steer clear of the more expensive, prosumer d300. between the d90 and the full frame d700, the price point on the d300 is just not worth it.
if you want to go the canon route, i'd have a look at the 50d. i don't know much about canon lenses, so i can't make a recommendation. i can say that, at least for me, the attraction to canon is that they have up to date wide angle, af prime lenses. nikon has yet to keep up, and the natives are restless (we'd love to have a 24mm f/1.4, but our cries fall on deaf ears over in nikon land).
there's some color rendering differences between canon and nikon which are mostly subjective. i tend to think nikon is more accurate and canon is warmer. you can find sample images on the web if you really want to see a comparison. but with all the in camera options and photoshop abilities, those differences are negligible.
there are also some feature differences. for the last couple decades canon has been the sport photographer's camera, for its speed, mostly. when the d3 came out, for the first time in a long time canon shooters were switching to the d3, especially for the olympics (if you look at pictures of the photo pits, you'll see a lot more black lenses than white). neither the d90 or the 50d will perform like the d3, or the 1d mk3, but they're not supposed to.
remember that pixel count doesn't really matter these days. 12MP is plenty to print 8x10s. hell, you could actually print billboards if you wanted to (billboard resolution is much lower since you're viewing them at greater distances, natch).
i've steered clear of the other systems like olympus, sony, konica minolta and pentax, mostly because i don't know much about them. the new high end sony is supposed to be pretty nice. i'll go ahead and stand behind that d90 rec.