I liked the 17-85 IS but got quite annoyed with it after only a few months.
it gets slated in most reviews. the wide angle verticals drove me crazy!
17-55 IS would be a great move, Not sure how great the 15-85 is. a step forward from the 17-85 but I'm not sure by how much.
if budget is an issue the tamron 17-50 is nice. or 28-135 IS if range i more important to you than wide
I wouldn't put a 18-55 IS (even thought it's well received) on a 50D as I think the camera deserves more personally
p.s chaz? i think your link is incorrect