Check out www.dpreview.com's recent review of the Canon XSi/Rebel 450D. It states that the 450D is clearly pushing itself above the entry-level DSLR market of its predecessor, incorporating much of the 40D's internals. Plus there are features the 450D has that the 40D doesn't. Here's what they say abut the new sensor:
"The new sensor is superb, and from a resolution point of view puts the EOS 40D to shame without losing any of the high ISO performance that has been Canon's trump card for so long."
Of course there are other reasons to get the 40D, but the 450D is close enough to the performance of the 40D that you could get the 450D and save half a pound from your camera body and a few hundred $$.
Also, Nikon should be announcing the D90 in June, the successor to the D80 and competitor to the Canon 40D. My understanding is that Nikon has some really great kit lenses compared to Canon (check reviews from www.kenrockwell.com). According to Ken, the Nikkor 18-200 zoom lens has replaced almost all of his other lenses, including his 50mm f1.4! Canon does not have any lenses that cover such a range with such good quality at that price, so you might get a lighter setup with a Nikon D60/D90 and the 18-200mm lens, rather than the Canon 40D with two or more lenses.
Ken also mentions the Nikkor 18-55 and 55-200 are just as excellent, so since you are on a budget, you could get the D60/D90 with kit lens and add the 55-200 later ($230).
EDIT: just saw you were referring to the Canon f2.8 lens. That lens is in another league (and price point) by itself, and you obviously know more than me about what you are looking for! To answer your actual question, Ken doesn't give the lens the best review, but then again he's partial to the Nikon camp: