I would not count on to many people dumping their XTI's .
The XSi uses sd cards which means the CF cards won't work in it . The largest SD cards I have seen were 2 gigs , but since My ZR85 is only a 1 meg in picture mode 2 gigs is enough .I haven't really checked on them . You jump to 12 and you cut down numbers of pictures on a card . I have 2 - 16 gig + 8's , 4's CF's etc .
I didn't even consider the 40 D when it came out . My money is going into better glass for a year or so .
The thing that will get me to upgrade is when the burst rate goes up . The camera will use CF cards , EFS lenses , maybe a selector for UV/infer-red pictures . [All that for less than 1000 and I'll order now .]
To me unless you can count on a drop of at least 20% , there isn't a reason to wait . Saving the money to buy is one thing , but waiting to save 50 or 60 dollars is a waste of time unless you can't afford it - " if I save to 500 I can buy in April . "
I put my XTI on a 0% for 9 months CC [at Circuit City - I wanted to say this is broke to a person without pressing 1 first if I had problems .] .