Depends how much money you have.I just thought, that as I was getting that anyway, I might as well take it below with me.
Just the Ikelite case with a standard port for the 450D will cost you double what a Canon G9 AND the housing costs together. (around $1200 vs $600) So even if you have the 450D for topside already...it's still cheaper to get the Canon G9 exclusively for U/W. (or several other P&S choices), than it is to house the DSLR.
And then remember that many say...it's not IF you flood your camera, it's WHEN. Now in honesty I'm not sure how true that is - I've never flooded mine....BUT....many, many people do - and a digital camera is toast if that happens...probably the lens too. You can mitigate with insurance.....but still. In my case maybe I should better say...I've never flooded it YET!