I was in this same situation a couple of months back. I went to my local shop to buy a D40 but I left with an XTi. Personal preference and a few feature differences. I had the XTi for a couple of weeks and started hearing about the new XSi.
I returned the XTi and waited for what seemed like an eternity (2.5 months) for the XSi to be released. Now I have the XSi and I love it. It's easy to use, has great lens options and is a camera that I can grow in to. Once I outgrow it, I would still keep it as a knock-around camera.
It takes great shots and I'm very pleased with it. In all, I think you would be happy with a Canon, Nikon or even the new Sony - it's a personal preference. I like Canon, I like their lens system and I like a few of the features on the XSi, so it was a no brainer for me.
Good luck - make a decision and start taking pictures! Don't get in to analysis/paralysis. Buy a camera and start shooting!