I was just about to start a similar thread so I will chime in here. I got an XTi about 2 months ago but only had about a week's worth of shooting time. I was looking at the XSi and noticed it's actually a bit taller than the XTi. I think the image quality on the XTi is pretty good and I'm sure the XSi's image quality is a little bit better due to a new image processor.
The XTi is still going for around $580-615 while the XSi w/kit lens can be had for about $670 shipped on Amazon. So I would definitely go for the XSi. I just checked out the XS today and it looked cheap. And while I like the layout of the XTi better, the XSi should be a better camera function-wise since it effectively replaced the XTi.
I think if you won't be able to carry an SLR around then just go with the G9 or just a regular P&S. If you're really looking at an SLR then you should look at the Nikon D80 as well. I noticed that the D80 has better image quality than the XTi and XSi (based on pictures I've seen online and I don't think they had much processing done).