This doesnt make sense for me! 24 and 28 at 2.8??? Why???
Maybe they renew all primes with is?
My guess is that this is their way of making up for the lack of APS-C primes, and to replace the old 24 and 28 without endangering 24L and 28 1.8 sales, with IS there to help compensate for the lack of speed. The 50 1.8 mk1, 15 fisheye, 24/2.8, 28/2.8, 35/2, and the 135 SF seem to have very similar designs; if canon wants to replace them all it'd make sense the 35/2 and 135SF are next. No idea if they are though.
As for the 2.8, fast wide glass is hard to design and expensive, and again, may damage 24L and 28 1.8 sales,
though there's always the chance the 28 1.8 is being replaced too.