ok, what i'm going to say is not going to be popular. I do own the 35L, 85L and 135L portrait lenses. And I would recommend you to just get an 85/1.8. I bought the 85/1.8 "for fun" as part of a double-rebate thing from Canon, so the lens was essentially "free" and it has sorta displaced all my other L primes.
For portraits, if you desire to photograph wide open at 1.8, I find that the slight softness of the lens actually flatters your subjects. And at f/4, the 85/1.8 is very very sharp. In the studio, I usually use the lens at f/9 or so, and it has proved extraordinarily sharp in any case.
For the most part, I really don't see the need for an 85L for photography. I own it, and its just this huge chunk of glass that I hardly use anymore. Its nice I suppose to stare that huge amount of glass, but really, if you are going to be photographing with a 1Ds3 and an 85/1.2 II all day long, vs a 5D and a 85/1.8, I'll gladly take the lighter combination. Its from personal experience, outdoors, indoors and in studio, that I think the 85/1.8 is so underrated. Its very great for its price. Small, light and useful for so many things.
http://img87.imageshack.us/img87/3268/img3848bn2.jpg
- I focused this shot on her teeth, and not her eyes - you can roughly see that her teeth are where the center AF point of the 5D should be. She just wanted a tiny thumbnail print of this - so absolute sharpness wasn't an issue. (P.S: She's a US Senator's daughter too...)