I'm in a similar boat... my 17-55 has sat in my lens cupboard for the last 6 months (since i got my 5d)... I was saving it for my back-up camera, but now i'm thinking of selling it.
My wife is getting a 100D and I was thinking she might use it with it, but attaching it to the 100D makes the combo so heavy/unbalanced i know she won't like it much.
My 450D is more use as a paper weight... it is so bashed up and crappy (compared to the 5D) I don't see much point keeping the 17-55 for it, since i have barely used it anyway...
I vote sell it... That is what i'm going to do, and put the money towards something else (I can get a 24 2.8 IS for not much more than the re-sale price... A plus for the 100D/wife and my 5D alike).