Here are the two scenarios I can suggest
first, get the 10-22 for your 40d to see how wide a wide angle lens can go. Then when the time comes that you'll be getting your FF body and decided to let go of your 40d, sell the 10-22 then get the 16-35L
or you can go straight to the 16-35L but mind you, the 16-35L on a crop body isn't really that wide. the good thing is you already have your lens. A friend of mine invested on lenses first. her first camera was a 350d and now that she's done setting up her lens lineup that she now upgraded to the 5DMKII. 16-35 + 24-70 + 70-200 f4 ISU (it would have been better if she went for the trinity lens but she said she can't take the weight of the 70-200 2.8 ISU)
for me I'd go with the first option. at least while waiting for the time I can get my FF body, I would be able to test if having a UWA lens is good for me and my photographic needs. so that by the time you go FF, you're already decided if you want an UWA or go for the 24-70 2.8L because 24 is already wide for FF.