I use a 450D for editorial works these days, cause I'm still waiting for a D800 or D3x to come out, or maybe just go get a D700 if I can't wait.
About myself, I shoot big assignments with rollei6008II with a dslr as digital polaroid; and for small deals I just use a dslr. I find that mostly a 450D is enough for editorial works, perfect for half page or smaller, and good enough for Full Page, if you take it really careful, I mean shoot carefully to prevent cropping too much in the post; and retouching carefully to prevent lose the details.
The positive part of using a 450d for assingments is that it's light and small, and cost less than perhaps any other choices, and good IQ most of the time, I mean good enough for the clients.
The negative part is that you just don't crop, if you want a kind of composition, shoot it that way, don't wait to crop in the post, cause it loose IQ a lot. If you crop half size of the original, it will be totally unpleasant to enlarge it to magazine full page.
I'll recomend buying a FF body anyway, since you use p21 as your primary gear. dont think about aps-c sized bodys. My experience is that a 450D is not that good as a 5D, especially when you enlarge to a size like double page magazine. In that case the 450d just doesn't work.
oh I forgot to mention, that when your clients see you 450d in your hand and ask if a 5d is good enough as a birthday present for his girlfriend, you are f***d.
I have a P21 back and I do have a spare Mamiya body but I don't want to spend out on a spare back so I was thinking of getting a 5D as a backup camera. I remembered a review of Michaels saying he thought the 40D was as good as the 5D and it seems the 450D is as good as the 40D and therefore also the 5D, with regards to IQ that is. Maybe the 450D is the perfect if a little unexpected, backup camera? Am I missing something or can I get away with saving a bunch of cashola?