I own a Rebel XT and 5D, and researched this topic a bit myself recently. I used the DoF calculator atwww.outsight.com/hyperfocal.html
to find that, for example, using my 85mm f/1.8 lens wide open on the Rebel XT focused at 30 feet (360 inches) gives me a DoF of 30.5 inches. Using the 135mm f/2.0 lens on the 5D will give me almost identical angle of view, and the DoF when focused at 30 feet is 30 inches using an aperture of f/2.8. Of course, I can then open up to f/2.0 and get an even shallower DoF (21.4 inches) or stay at f/2.8 and keep the extra resolution.
Unfortunately, I can't really comment on the difference in background bokeh because my informal testing was at my son's soccer game. Shooting any type of sports you just never get exactly the same shot twice. But I was very pleased with both camera/lens combinations for that use, and for the prints I'll make of that - likely no larger than 6x9 - the Rebel XT is sufficient, considerably lighter (2.1 lb versus 3.6 lb), and less conspicuous. So in the future I'm sticking with the "eight is enough" Rebel XT solution in that circumstance.
I must add that I'm very impressed with the results from the innocuous-looking little Rebel XT and 85mm f/1.8. Both are inexpensive little jewels of performance at $1,240 total versus $3,900 for the 5D with 135mm f/2.