Ry,
Three weeks ago I was in the same boat.....Went into a store and held the 40D (great camera!), then held the XSi (450D elsewhere in the world). Back and forth.....
The 450D viewfinder is much improved, I don't need dual wheels or to spend anymore than necessary. Plus I'm lazy and want the lightest weight possible camera that will do the job. I know that's more my personal choice and many want or need a heavier camera.
So I bought my first of what will likely be 2 Canon Rebel XSi bodies for the next year or more
I liken buying capable low cost RAW producing dSLR cameras to when Nikonos V cameras became the "standard" and were affordable enough to have 2, or 3 or more...Insure it, use it and IF it breaks pull another out and keep shooting......
Problem is, that theory has yet to prove itself! I have progressed through the Canon Digital Rebel series including the original Digital Rebel, Rebel XT and Rebel XTi I have yet to break one exceeding 20,000 "clicks" on several bodies. I know several shooters with the original Digital Rebel having upwards of 75,000 "clicks" and still going strong.....
So far the Canon Rebel XSi (450D) images even at ISO 800 and 1600 look great! Even the new 18-55mm IS lens is sweet and vastly improved over the previous model. Reviews rate it almost equal to the$1,000 Canon 17-55mm f2.8 lens performance! Here in the U.S it only cost $100 more to get the IS kit lens with a 450D. So I'd advise getting it if only for a lightweight travel lens.
Ikelite's housing with eTTL2 will start shipping in a week or so. I already sold one package to a customer with it. Plus I'll be getting my own housing soon 
The 450 D feels very solid for a "consumer" camera....
Just one lowly photographer's opinion....
dhaas
www.haasimages.com