I think I'm with Richard Lane on this one. The 35mm f/1.4 would probably be a better choice than the 85mm f/1.8. The DOF at 85mm will be thin enough that you'll often want to stop it down a bit from f/1.8 anyway, making it an often redundant lens. The 35mm is a bit faster. It's a usable focal length for indoor shooting, and it's closer to a normal (non tele, non wide) focal length on a crop camera. Of course it's a fair bit more expensive too. Several people here opted for a Sigma 30mm instead of the Canon 35mm, which brings the price back under control.

There are excellent reasons to take the 85mm f/1.8 anyway, even if it is redundant. It is significantly lighter and smaller than the 70-200. There will be days when you won't want to drag that beast around with you.

I'm not sure what the Chromatic Aberration is like on the Canon 35mm or the Sigma 30mm, but the Canon 85mm gets a fair deal of purple fringes at the slightest provocation. I'll let the others who've used both to fill in the details here. If you're going to use Lightroom it can, apparently, magically make the purple go away.