Some Target stores have had the Rebel XSi + 18-55 IS kit on clearance for $450 this month. Good luck finding one at that price, though.
Most DSLR camera bodies don't stay around for very long (except for the ancient low-end Nikon D40, which is three years old). The Rebel XSi has been out for a year and a half.
Consider this a vivid example of why it makes sense to allocate a larger portion of your budget for good lenses, rather than paying a premium to have the latest and greatest camera body that brings only incremental improvements over the previous model.
Of course, if a new model comes along that has a quantum leap in capability or features over the lower-priced ones and those features matter to you, then it would make sense to get the newer and better camera.