telf:
I was in a similar position pre-X-mas and here is how I decided and why - note, I have little DSLR experience, but very recent DSLR shopping experience (just a disclaimer).
I was looking at getting a D40/D70 or a used D80 if I could find one cheap enough (I would group all of those together as 'Nikon') OR a Pentax K10D. The Canon's were not in my equation because I didn't like how they 'felt.' If you go and handle the D40, K10, and 20D you can see the differences in body layout and design. Don't underestimate how important this is. Step one for you, go hold the different cameras you are considering.
I ended up getting the K10D. I would have been just as happy (IMHO) with a Nikon.
The pluses for the Nikon are mostly related to brand name. You can be sure Nikon isn't going out of business, they aren't going to stop making digital camera bodies, they aren't going to stop making lenses. Of course, it is unlikely that Pentax will stop either, but it is worth considering. If you have Nikon lenses already, then it would be a no-brainer. Read Thom Hogan's reviews of all the Nikon products - he seems to be "the guy" in a lot of ways and when I was doing research he seemed pretty unbiased:
http://bythom.com/
The pluses for the Pentax were numerous. Weather sealed body was the main one for me - we do a lot of hiking/camping/skiing/etc. and having that extra bit of protection for the innards of the camera is nice. The K10D has a more megapixels (this did not make or break the deal for me - worth mentioning, though) than the D40/70. The K10D has in-body image stabilization (not as good as the in-lens on the Nikon, but it works on every lens that you put on it). The K10D works with (almost) every lens that Pentax has ever made - see a pentax lens at a yard sale? Pick it up, you can almost definitely use it. That gives the K10D a huge lens selection (although some will be partially or totally manual).
Ultimately, even with all of the technical advantages, I was still back and forth over the D70 or the K10D. Ultimately my wife got me the K10D. I had already decided it was what I was going to get, though, because of a Thom Hogan review of the D80 - he said that if he was starting from scratch now, he would by the K10D. To me, that was quite a ringing endorsement.
More than my .02, but I hope it helps.
/matt