For the crop-frame the EF-S 15-85mm is a nice versatile lens, and would pretty much replace your 18-55 with an slight all-round improvement in image quality and build, with the plus side that it goes a fair amount wider and longer than the 18-55mm. IMO, if you are seriously
thinking about upgrading to 'full-frame' later on though, it'd be better to just use what you have for now and save the money for better EF lenses down the track... With how good photostitching software has gotten these days, an ultra-wide-angle lens isn't really needed unless you're chasing the stronger perspectives that only a UWA lens can deliver when you get up close to things. That and most of the UWA lenses available for the crop-frame bodies look pretty average in terms of their IQ, particularly towards the edges of the frame, lots of diffraction an CA from the sample shots I've seen.
One other thing - I have a EF 16-35mm L II to use on my 5DII. While I love it now, I hated it when I was using it when I still had my 7D. The wider image circle meant I was forever having bright light-sources throwing light down the lens and flaring in the picture because the lightsource (sun, streetlights, etc..) couldn't be seen in the viewfinder - you'd only realise afterwards.
So while it is a technically better lens than the 15-85mm, I would say it was a worse lens when mounted on the crop-frame body - the 15-85mm allowed you to get the framing/composition you wanted more easily and more often, as you didn't have to allow for things outside the frame that would otherwise effect the image...
That's some of my experiences that spring to mind anyway.