to tell you the truth,.. reading these forums, and lookin at reviews on the 18-135, i heard alot of mixed reviews on it.. the 17-55 should definitely be ur primary lens on your 60d tho IMO..
but the funny part is i went to B&H the other week and i was playing around w/ the 60D and the pictures that came on the back screen were actually very nice and i was surprised it was from a 18-135.. having previous knowledge that it wasnt that good of a lens.. then again its hard to compare a preview picture on the back of a camera compared your computer screen..
i'd say put ur 18-135 on your xti, as i presume it is your back up camera now.. and u can just carry combo on either days you dont wanna take serious pictures or wen you dont wanna worry so much about hurting your expensive equipment..
have your 17-55/55-250 as a nice combo ..
its funnny i say that b/c im acting like ur xti is like garbage.. which it is obviously not. but wen it comes down to having two cameras, u have to have two different purposes for them.