Scott,
The sensor in the Rebel XSi is actually more MP than the 40D. Many won't consider it due to no second command dial and yes, as James says he viewfinder is a bit smaller which is reflected in the price. That said, the XSi viewfinder IS quite a bit better than the XTi, XT and of course original Digital Rebel.
It's an incredible powerhouse producing RAW files of 15 MB and more. Put good glass (or even not so good glass at the right focal length and f-stop) and it can produce professional results.
I have two of these lightweight wonders making travel much easier
YMMV
dhaas