IS is of no use for concerts. Have'nt you noticed that these people move
around a lot? (like they have ADHD sometimes)
IS is only usefull for static subjects.
If you want a zoom with large aperture and the sharpnes of a prime? There
can be only one!
EF 24-70 F2,8 USM
It also fills your 'gap' very nice.
kr
Aad

$Not decided completely yet which way to go, but I will be looking for a lens
$that's suitable for concert work, where flash is verboten, so a wide
$aperture is one of my major concerns, with image stabilisation if possible.
$
$So it's looking like Canon will win the day, unless there is another
$manufacturer that offers IS?

  There is only one third-party image-stabilized lens available in
the Canon EF mount, and that's the Sigma 80-400.  So yup, you're looking
for a Canon lens here.

  Another lens you might consider, given a desire for a lens which is
fast and has IS, is the EF-S 17-55 f/2.8 IS USM.  It's supposed to be
very good optically; its use of aspherical elements and low-dispersion
glass is similar to that of the L-series wide-angle zooms, and tests
and reviews of it show that its optical quality is what you'd expect
from those technologies.  It lacks the letter L and magical red ring
because it's an EF-S lens and can't be mounted on pro bodies (and
in turn, since it's not an L, it isn't dust- and water-resistant,
and its build quality is that of a high-end consumer lens, not an
L lens).

  But one thing to keep in mind given your stated use is that IS
doesn't help if the subject is moving.  In fact, rather the opposite.
IS lets you get steady handheld shots while using *slower* shutter
speeds than you could without it.  A slower shutter speed gives
the subject more time to move around, causing them to get blurry,
so you might get nice, sharp pictures of the stage with blurred
performers, unless they're not moving while you take the picture.

  Drop by http://www.photo.net/learn/concerts/mirarchi/concer_i
for a tutorial on concert photography.  It's from the old film days,
so there are some technical issues which will differ (such as the
usability of high ISOs on modern DSLRs vs. film, and of course
the difference in focal lengths due to the crop factor).  But I'd
particularly draw your attention to the discussion of fast zooms
vs. primes; the article's author does a good job of explaining
why both have their places in concert photography.

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It is my normal daytime walkaround lens. Some copies apparently have front
focusing problem but I have not experienced that. At widest focal length
slight double distortion (barrel in the middle and pincushion at the
edges) is noticeable if you test it critically. In practical use, I have
never noticed it.