One thing no one has mentioned is ergonomics, Nikons are usually a nicer camera to hold, buttons laid out well. They were along way ahead of Canon on the ergonomic side for some time but Canon have become alot better in the time since the 350d came out. Now i would say they are comparable but nikons are still nicer to hold, which is annoying.
I am a Canon man, one reason was because when i got my first camera (the trusty 350d), Nikon didnt have a camera that could touch it (through reviews etc). But I never liked the Nikon brand, the cameras are fantastic but i wasn't compelled to buy them. But I was a young 16 y/o when i got mine, now im a professional graphic designer and photographer and i see why I went down that road (with successful brand identity and a compelling product), I am also glad that i went Canon. There is something compelling about the Canon brand and they offer a full system that is very difficult to beat. But there is not alot between the two. The fact that you have come to the Canon rumors site tells me you are intrigued by Canon, which is great.
It is also about what your used to and what suits your style of shooting. I cant stand the UI in the Nikons, one reason I went canon in the first place. Every time i go through the menus i feel they could do such a better job nut that is my opinion, nothing on the cameras. But these are things you have to try because my opinion may not be shared by you.
But in recent times Canon have been slipping, there is a serious problem with the brand identity of their cameras, it used to be very simple xxxD amateur, xxD semi pro, xD pro, now i feel they have made it difficult for the consumers in an attempt to compete with nikons better laid path for future development. But this is probably because Canon have a map that they feel is best for them but we haven't seen that complete transition yet and the development of this road is an interesting one. Now we have beginner, semi amateur, amateur, semi pro and pro cameras, and there is not alot of difference between the beginner to amateur cameras IMHO. Now anyone with any knowledge of the Canon brand, technically minded or a Canon follower will understand the map ok... but for beginners looking to upgrade from a point and shoot they have definitely made it difficult.
This is not necessarily a bad thing, but what is bad is Canons lack of direction in upgrades, they seem to be abit stuck in terms of a revolutionary camera, the 600D and 60D were massive disappointments in my opinion, great cameras but nothing new, just an upgrade. Now you cant expect brands to produce revolutionary technology every upgrade, but for the current owners of previous products it is difficult to justify. Most of Canons revenue will come from new buyers, so that is probably why there have been minimal upgrades recently, especially with the up and coming Digic 5, basically making the most buck from the technology before it is superseded. The 7D would be a great investment but nicer ISO performance would be nice. The 40D is known to have better ISO performance than the newer cameras due to packing too many pixels in etc. The 40D is becoming long in the tooth.
But just as Canon has a few problems to iron out Nikon have had some pretty big dry patches. When the 5D was launched Nikon were so blown out of the water it took them along time to produce a camera to compete, and alot of very loyal nikon users moved over. This was a pretty thretening time for Nikon but they did their homework and now have a fantastic range, but then Canon was definitely the brand to own because the Digic 4 processor was so advanced...
Its swings and roundabouts, one company wont be better than the other constantly especially as tech is getting very tight, how many mega pixels do average consumers need? how many DSLR owners have an A3 printer?? I bet less than 10%, every crop camera on the market can now make a good A3 print regardless of brand. We are on the verge of something new and cameras are becoming swiss army knifes with video etc Now the mirrorless cameras are threatening the DSLR market. It is an interesting scene that is unfolding and Canons dramatic exclusion from the mirrorless market may be an inclination of their direction. I feel like something big is coming along from Canon and all there resources are going towards it, lack of decent upgrades, tech etc
Canons future in my mind is very interesting and im very happy to be a Canon follower and user. If a brand can get you excited about the things it is selling half the battle is over. Look at the Mac perfect example of selling a lifestyle brand to improve your life and the way you use technology.
My ultimate advice... go Canon you wont be disappointed and the future can only get better... but good luck with wherever you go with your decision.