My thoughts are that the DA*50-135 is an *obviously* great choice for concert photography, *if* you don't mind the price or weight or size. Me, I'm a cheapskate, a weakling, and self-conscious about taking pictures, so the 50-135 is only an occasional blip on the outskirts my radar.
After shooting a bunch with 50 and 135 primes (that cost me under $100 combined) and deciding one was too short and the other too long, I bought an M100/2.8 (for around another $100). I feel I'm pretty well set now. The M100/2.8 stays on my camera most of the show, although I'll switch to a 50, 40, or 28 when I want something wider, which I don't all that often. Of course, with the 100, most of my shots are closeups, which is what I tend to like most. If you prefer like wider shots, then something shorter makes sense. And if you feel the need to take lots of both, then by all means, the DA*50-135 is great (not that I own one - but I've handled one, and seen plenty of pictures from it). Although something like a 28-75/2.8 should also be considered if you think having something wider than 50 would be more useful than having something longer than 75 - and it's quite a bit cheaper, smaller, and lighter than the 50-135.
All these focal lengths are also relative, of course, to how close you expect to be to your subjects. I'm dealing mainly with jazz clubs that seat around 100, and I try to sit a table in the front. At larger venues, you need correspondingly longer lenses unless you have a press pass. And at places where you're basically standing up and in danger of being hit in the forehead by a sliding trombone (substitute guitar or other intsrument applicable to your genre of choice :-), you need something that much shorter than if you're sitting comfortably at a cocktail table. So again, assuming you're OK with the price/size/weight of the 50-135, it's practically a no-brainer as far as I am concerned.
Oh, as for f/2.8 - sure, we'd love faster, but there are relatively few choices in useful focal lengths. At least, few that are even remotely afforable. The DA70, FA77, and M85/2 could all be worth considering, as would trying to get by with a 50 - although that's just *too* short for me almost all the time. So I open up to f/2.8, set my shutter speed to 1/30", and take whatever exposure I can get, knowing I can save it in PP. It worked on my DS, using ISO 1600 rather than ISO 3200 (it actually works out to the same thing, though, since ISO 3200 is just a software push anyhow on the 6MP cameras), and it works on my K200D which produces no more noise than the DS when viewed at the same size. Some say the K10D isn't as good as the K200D, but it's certainly worth a shot.
Some examples from tjhe M100/2.8 (apologies to those who have seen them before):