I will definitely have a look at the 15-85mm. I just have two concerns about it. I'm seeing myself possibly with a FF camera down the road and then the 15-85mm would be useless. The other thing is that f/5.6 is really slow. Do I still get a nice OOF blur at 85mm?My suggestion is to choose a format and commit to it. Don't get caught halfway in-between trying to hedge your bets.
You just bought a 60D. So get the 15-85 now. You'll get lots of mileage out of it. You'll get your money's worth. In a few years you can re-evaluate and think about selling. Buying lenses is not like marriage, it's not a forever commitment.
Or else, bring back that 60D to the store and get a 5D with 24-105 kit lens now.
As for shallow dof with a standard zoom on a crop camera: neither the 17-50 nor the 15-85 will produce much selective focus except at close focusing distances (headshots and closer). So that's not really a consideration. If you want selective focus then you need a fast prime (e.g. 50/1.4 or 85/1.8 ). The reason for a f/2.8 zoom on crop is light-gathering capability. f/2.8 zooms are the way to go if you're shooting indoors a lot (with or without flash).