Jubu, i was thinking of a move to the 40D, considering the prices are dropping, but I really had to think hard about what I will gain and what I would loose in the process.
The XSIs sensor is one of, if not the best crop sensor than Canon currently makes - the dynamic range, highlight recovery , noise performance, etc are superb.
The camera settings display screen is probably one of the best features I have seen on a camera in a long time. IMHO after using pro bodies for years, this blows everything out of the water and should be a requirement on pro bodies. I can operate the XSI faster than I could even my old D200 because this provides a one stop shop for settings view and change.
Size/Weight. The XSI can hang around my neck all day long and it's like it's not even there - of course a pro lens can change things, but still the lack of weight is a big plus. Smaller bags, portability, more candid/stealth - these things are big pluses.
KISS - the XSI doesn't have a ton of buried menus, overwhelming buttons or dials (which are simply one more thing that one can accidentally press/change). As a long time shooter it has exactly what one needs, nothing more, nothing less.
The 40D/50D are 'cool' cameras and offer more appeal because of their status as pro bodies, more than their features. At one time, the differences between consumer and prosumer was wider, however the margin has narrowed significantly over the last few years. I have read dpreview's review of the 40D/XSI more times than I can count, and every time my conclusions are the same, there just isn't anything significant to gain. That doesn't mean I wouldn't consider the 40D if a good deal came along, cause like you I sometimes am bitten by body lust. I think your decision is fine either way, this is just food for thought....
Be careful with Adoroma, I sent them a few lenses in a while back. Those fast talking New Yorkers quoted me one price over the phone and after receiving my goods told me another price and even had some attitude when I asked why the price changed. Their trade-in prices are higher than others.

. My one hand under the lens style of shooting may be a factor as well I suppose.
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