Matt, thats awesome on the 5D thats a nice camera. Remember that its a full frame camera though, so if you're going to be buying lenses I'd op for the full frame lenses that will fit the 5D as opposed to the crop lenses.
I ended up selling my sigma and getting a tamron in the same focal range. I think its a much sharper lens. There's still the 2.8 factor, but I have never really had that many issues, however, I'm a burst shooter. I agree with what you're saying tho, and the 135L is a great lens. If you're looking at portrait lenses the 24-105/f4 is supposed to be a great all around lens, but its f4, so its pretty crappy in low light, however it has IS and L glass, so as long as your subject isn't moving too much, the IS should help the steadiness of your shot.
Ed, did you have the 26-70 or the 24-70 sigma? Big difference between the two...
Well 2 things make for a good portrait lens and that is the focal length and speed. As we all know you need a fairly long lens or a fast lens to blur a background and the 135L is both. I know the 5d is full frame. that is one reason that I am justifying the 135mm. If they made a 100/2 that was L glass I would probably buy that, but they don't and the one they do make is crap. With a crop sensor the 135 is a bit long for portrait work, and probably even for concert shooting unless you are pretty far back. With my rebel and 50/1.8 if I am 10' back from stage I still can't get more than a single person in the viewfinder unless I shoot at an angle, but then because of the DOA inherant in the open aperture you can only focus on one person... then if you are shooting a slow speed you basically get a blunch of blurred people around a perfectly focused mic stand!. Concert photography is NOT easy to do really well.
If you look at that pic I posted, it was shot with bounce flash @ 34mm/2.8 and I got a touch of blur but not enough for my liking. In this case with my rebel it was shot at about "normal" on the zoom because of the crop factor. If you get too crazy with a fast telephoto , you will blur the nose and ears trying to focus on the eyes. With a crop sensor the 135 would be too big to shoot even a headshot with less than 15 or 20 ft I imagine, unless you can stop down quite a bit. For the record... the ability to blur with a telephoto is not based on the perspective but on the lens length. so even though a 85mm on a crop sensor will yield about the the same field of view that a 135 does on a full frame, it doesn't have the same ability to blur. This blur btw is called "bokeh".
And freelunch is right... if you are willing to shoot manual film cameras you can do low-light cheap. I just sold an xg1 body with a 50/1.4 for $45. However, that all being said, film is a PITA. I've recently started back to college as a photography student and they require us to shoot slide film. It is very hard to find a place that can even develop slide film. Many places won't even send it out anymore. There is a reason that stuff is dirt cheap. It is all basically worthless. I just bought a canon 1n film body that is 4 years old with about 10k actuations on it for $250 shipped. It was a $2500 camera only a few years ago and it is a joy to shoot, but shooting film is a joke. With flash media dropping in price every day and camera manufactures increasing resolution twice per year you can safely say that film is all but gone. There will still be medium and large format film in use for a few years but 35mm film is going to fade into the wind mighty soon. Photo labs are closing left and right because they can't make any money.
Matt