[JAHphoto]

I didn't actually transition, went straight from film to 5D, but I did get a 30D later.
It really depends on what kind of photographer you are. For me, I like to do all sorts of stuff, but a lot of wildlife and landscapes.. so I like the extremes, long and wide.
Kinda poses an issues, but I decided to go the 5D route.
At the time my justification was "Well, the 20D is 8MP, so I could just crop anything from my 5D and have similar quality"
True at the time, but with that 40D, at 10.1MP, a crop from your 5D is less quality.

But when I put my 17-40 on my 5D, its all worth it. You can get a fisheye, whatever, it cant compare to a great quality wide angle on a full frame.
So for me, its worth it.

In this scenario, would you be selling your 40D? If you're not, maybe just start with a 5D and a 17-40, and keep the 40D for using the 70-200 and such.
But it really depends on what you do. If you shoot sports (which I do, but I get a D200 with 2.8's to do that with) then you need that 70-200, and a 5D really isnt well suited to that. It will do the trick, but not as well as 40D.
For my artistic stuff, I don't need fast lenses, so I have my 100-400 to compensate for the lack of sensor crop when I'm using my 5D (and the wonderful 160-840mm on the 30D)