I am interested in camera recommendations with a fast shutter and which are able to focus well in low light. The camera will be used for indoor concert photography where no flash is allowed.
As usually in life, the best things are not cheap.
Given only the criteria of fast shutter and good focusing in low light, I suspect Nikon D3s is still unbeaten. It also has a quiet shutter mode, should you visit concerts with silent audience.
A Nikon AF-S 70-200 ED VR II 2.8 lens would be a good match for low-light concert photography. The big aperture helps your camera to focus in the low light, and zoom gives you more freedom of composition. It has weather sealing (just like the camera) in case you would ever visit an outdoor concert with sudden rain.
While the sensor of Pentax K-5 is also praised for its low-light performance, auto-focus has been Achilles heel for Pentax and while vastly improved on K-5, it still lags behind the big two. Also slowness of supersonic AF motors on their premium tele lenses is not encouraging.
Given a budget of around $1000 for the camera body, as you mentioned in a comment to one of the other answers, the answer is:
Any camera body you can buy for around $1000.
Seriously; in this price bracket, they're all competitive. There are some variations in focus speed in low light, but that mostly comes down to a tradeoff of accuracy vs. speed — and even then, for around the same price, the differences are going to be pretty small.
By "fast shutter", I assume you mean responsive shutter. Unlike with point and shoot cameras, that's really not much of an issue even on entry level DSLRs, and going up a step or two means it's really a non-issue. Cameras in the $1000 price range will have very fast shutters in another sense too, by the way — able to be as quick as ¹⁄₈₀₀₀th of a second is typical. If your concerts are outdoors and very brightly lit, that might be of interest. :)
So then, the important thing will be the lens, and the recommendation there will be very dependent on exactly what kind of concert and on your level of access. It'd be helpful if you could elaborate. But in most cases, there will be reasonable options for any brand and camera body.
So, you should look at a few $1000-range camera body reviews online. I prefer the subjective reviews to the technical, since the technical ones tend to exaggerate differences, because that's what drives people to those review sites. Find people talking about the feel of the camera, and see if that fits what you're looking for. Talk to your friends, too, if they're into photography. And then buy whatever you find yourself most drawn to. You really won't go wrong.
I routinely shoot low-light no-flash with my Nikon D7000 and 50mm 1.8. If you have a good raw processor (DxO Optics Pro in my case), the 7K set at 12,800 is quite usable for print. With good post-processing, even a zoom with F 5.6 would work.
Do you want a DSLR or something smaller? I would choose something with CMOS Exmor chip, which have very good low light photos. For example Pentax K-r (or older K-x) with some good lens. When it should be compact, then could be something like Samsung EX-1, Nikon P300, maybe Olympus Pen or the new Fuji X100. Or something between these categories - mirror-less "DSLRs" like Sony NEX or Samsung NX series. It depends on how much money do you have and what you expect. :-)
Probably Fujis X-M1 and other X-Trans sensor Fuji cameras. They have impressive low-light performance (not the autofocus, but the image result), even better then DSLR and even Full frame sometimes - check out image comparison on dpreview for high-iso. Colors are nice as well. But they force you to use JPEG at high iso, and I think you might have to do manual focus in low light. Fujis are also, in general, durable cameras (when compared to other cameras in the same class).
There are also micro four thirds camera (Olympus or Panasonic, or Blackmagic if you're into video), their sensors are less impressive when it comes to high-iso, but you can mount the great Olympus 75mm 1.8 lens (150mm in 35mm equiv) on them - preferably on an Olympus OM-D body because you would then have image stabilization (in-body). This lens alone is a good reason to look into that system.