Given the price drops we have already had, it would appear the XT is bordering on close out, the XTi may go lower but not until after you see the XT gone as the prices are fairly close right now.
Also right now, the XTi makes for a good deal as the very entry level DSLR, think parents buying for a high school student and not wanting to over spend as it could just be a fad etc etc. I haven't been pating a lot of attn to exact difference but it looks like about $350 difference maybe a little more between the XTi & XSi.
The XTi is sitting around $450 now and recently, Dell was selling the body only for $320 and $4xx (don't remember off the top of my head) for the body with the kit lens, which is a phenomenal price (lower then an XT usually is)
I think it will drop to the $400 range used/occasionally new once the XSi hits stores.