17-55/2.8 vs 15-85/3.5-5.6 is a popular question around here. I went back and forth as well, but eventually settled last summer on a 17-55 ($750 from eBay). I've been very satisfied with it any enjoy the f/2.8 capabilities, plus appreciate not having to deal with a variable aperture. I take quite a few photos of people at gatherings, which of course have low light and people...both useful to have 2.8.
Everything I hear about the 15-85 is good though. If your subjects aren't moving and you've got strobes, it could easily be a better choice with the added range/lower price/smaller size.
There's got to be dozens of threads on this I've seen over the last year...if you search there's a lot of people who feel strongly both ways in this debate that can hopefully help with your decision. I doubt you'd regret buying either, truthfully.
