Ober and Rob, you're both right - the biggest difference between the XS and XSi is the live lcd view finder.
And the XS actually has the "live view" but it's very low quality, and once you get used to using the regular view finder, you don't miss using the LCD. My brother has an XSi and I don't think they ever use the LCD view finder either. I guess what I'm saying is that the XSi is a waste of money.
I don't use the LCD vie finder either, but it is still a nice feature to have. When my wife uses the camera, she likes the LCD (the LCD is bigger than on the XS).
I don't think it is a waste of money. It does have higher pixel density, it does have two more pixels, it has spot metering, better AF, can use a wireless remote (a feature I love) and has an LCD shut off (proximity) sensor. I did quite a lot of research on these two when I was getting mine; I figured that I will use the extra features it offered - and I am. The $150 difference made sense to me.
