IMO, anyone who hasn't considered it isn't serious about their hardware. In fact, I'd point out that there aren't just two sides. I consider the smaller brands too.
But, not only do I have a glass investment with Canon, I also have a personal familarity with it, not that it woudln't translate into a reasonable familiarity with any brand, but I have never picked up a Canon SLR and felt out of place. Sure, some buttons are missing, there might be a new button here and there, and certain placement varies a little, but when I pick up a friend's Nikon (yes, I'm friends with Nikon owners), there is a different philosophy in place which is reflected in the buttons/switches, their functions, and their layouts. Its not amazingly different, but its different enough. There are times I like the Nikon philosophy better, but not usually.
I think if I had started with Nikon, I'd prefer it, just as I prefer Canon since I started with them. There is the old saying about "the grass is always greener". Sometimes the grass is actually greener, but that usually means more maneur. People starting off have a great chance to invest in the manufacture that best fits their hands and their minds. I wish all experienced people would push newbies into trying both/all cameras equally and getting them a great fit, instead of pushing them into their own choice.