For a walkaround lense, the 24-105 is pretty darn good on the 5D if you only can have one lense. Compared to the 24-70 it's also lighter by a fair bit.
I used it a whole lot on my 20D, but always with the addon of a 10-22 for the wideangle. With the 5D mk2 I got for my trip to New Zeeland, I rarely used the 17-40 at all. I got that one to replace the 10-22. And while I often used the 10-22 at 10, for some reason, I find that the 24-105 on 24 works well enough in most cases. I dont know how the 40D compares in Iso, my 20D was kindof bad on higher isos, while the 5Dmk2 works fine to 1600 and even 3200. Thats also why the 24-105 works well with 5D mk2 compared to the 24-70. Same thing with the longer zoom's too. Enough so I sold my 70-200/2.8L IS and stepped down to a 70-200/4L IS, mostly because of weight for the trip actually, but shallow dof is not an issue if I want it.
That said, the 24-105 have a problem when doing portraits. You will need primes if you do portraits with shallow dof in any way. It wont be at its best wide open either. And as has already been said, it could be sharper in the corners, but it not that much of a slouch if you step it down a tad.
But unless theres a special reason you want full frame, Voldenuit has a good point that 7D with 10-22 is a pretty good combination. But still... 5D mk2 with the 135/2L is pretty hard to match for a APS-C camera when doing portraits.
And the 5D mk3 is droolworthy. I actually wanted to wait for the 5D mk3, but spending 4 weeks in New Zeeland 2010-2011 with a 20D wasnt exactly what I could call optimal, so I caved willingly on the mk2, and it has served me very well for the last 1.5 years. Although I havent shoot much video with it. If you can wait A bit it'll probably fall from "new" price a tad too. And I would say that the differences between the mk2 and mk3 is worth getting the new one. But thats me...YMMV