personally i wouldn't bother in any f2.8 prime (except >200mm), you can get just as good images from recently-released zooms these days. (compare the photozone.de reviews of the 70-300L and the
300 f4L, they even say the prime isn't any better than the zoom (except for the wider aperture)).
there's only one reason i think anyone would consider it nowadays, and that's size+weight (or maybe cost).
I'm definitely hopeful that one day the wide-prime-nonL lenses will get an upgrade, but i'm not holding my breath.
Canon have this mentality of "if you can afford a 5d2 or 1ds3, you can afford a 17-40L or 16-35L or a 24/35 f1.4L, and if you can only afford an aps-c then you'll only want the 17-55 f2.8 and if you want a faster prime then you won't be able to see how crap our 20-year old lenses are on your low-quality aps-c sensor because if you cared about quality you'd buy a 5d2 with a Xmm 1.4L"
rubbish, and i'm still hopeful that since the release of the 7D canon might realise that even professionals care about image quality and are using aps-c. we still don't even have a 'normal' length affordable decent quality fast prime
- 35f2 is a bit squishy at f2, not worth the savings from 17-55 f2.8 for the extra speed.
- sigma 30 f1.4 is very soft on the borders.
- 35 1.4L is just too expensive (and huge).
- personally i'm scouring ebay for old manual focus primes in this range, they still beat the 35f2.
so back to original point: i see very very little chance of this ever being updated, f2.8 primes just won't sell well, unless it has the quality of an L in the same size/weight/cost bracket. If anything we might get a 28 f1.8 replacement, (i'd be happy with efs 28f2), or maybe a 35f2 replacement (efs 35 f1.4-1.8 would be better)