I have both the XSi and the 7D. I was a beginner when I bought the XSi a year ago and as I get acquainted to my 7D, I am more thankful each day that I started with the XSi. I've invested quite a bit in lenses and off-camera flashes over the year and I'm thankful that I had the extra cash (from not buying the 50D I was looking at then) to sink into my lenses.
I want to say here that I absolutely LOVE my XSi! It's never failed me and I get great quality photos with it. I shot my sister-in-law's wedding in June with it and I've gotten nothing but compliments on the quality of the photos! I only got the 7D because I needed faster fps (frames per second) to shoot my daughters' soccer games and cheer compititions.
In my most humble opinion, I would suggest starting with the XSi. Once you get familiar with that, then upgrade. The focusing system in the XSi isn't as complicated as the 7D's and as a beginner, the advanced focusing system in the 7D may be quite overwhelming for a newbie.
I would like to add that the 7D is an awesome camera but I think that you would be very overwhelmed as a beginner trying to learn to use it as your first DSLR.
Here's a link to my Flickr photostream. All of the photos there are taken with my XSi.
PhotogChic712's Flickr Photostream
Good Luck! Hope you let us know what you decided!