I'm quite excited to see an actual lens announcement from Canon - and well done to CR guy for predicting the 24-70mm L f/4 IS USM. 
As I don't plan to go full frame (at least not in the near future) - the 24-70mm L f/4 doesn't interest me so much. I'm very happy with my 15-85mm IS USM on both my 7D and 350D.
If the 24-70mm f/4 L has great IQ, I can imagine it will be popular as a less expensive / less bulky walk around (aka 'kit lens') for some folks having a FF, perhaps particularly the new 6D. (though I will say here that a possible price of $1800 USD is quite scary!) 
To me, the 35mm f/2 IS USM is actually a much more interesting lens... and.... in one sense meets a lot of the criteria I have for the 'only lens' that I'm really looking forward to, or - that is what I find 'lacking' in my current lens arsenal: a sharp prime at about 50mm (+/- a few mm).
The new 35mm has the following features, which are important to me:
- USM - my preferred AF mechanism, with FTM focus
- 4 stop effective IS
- 7 or more blade circular aperture
The size of the 35mm f/2 IS USM is still small enough. I guess any 50mm would be a bit longer... possibly could have 58mm or 67mm filter size. (I hope if a 50mm prime comes out, that it has a 58mm, 67mm or I'd even 'put up with' a 72mm filter size - as I have filters for these sizes). 
I expect the IQ of the 35mm f/2 to be very high (better than the 40mm f/2.8 pancake, which has received lots of praise, particularly for its price!)
My 'only' concern is that the 50mm might be a f/2.8 lens.. (no, please, no!) I had already decided that f/2 is perhaps the limit that I'd be happy with for a 'fast prime'. Obviously f/1.4 would be ideal. With f/1.8 or f/2 being 'good options' - particularly if IS is included. 
There would be some situations that 35mm is more suitable than 50mm (eg some group shots, etc). However my own photography style, means that I really like the 50mm aspect on an APS-C (giving the equivalent of 80mm in FF format).
So... now that the 35mm f/2 USM IS has come out (after the 'earlier this year' new 24mm & 28mm f/2.8 USM IS lenses) - I have more hope that a 50mm f/1.4-f/2 USM (possibly IS) lens will be released in the near future. I plan to go to a bricks and mortar shop and use the 35mm f/2 USM IS sometime.... see how I like it, get a feel for it, and hopefully see a new 50mm prime in store sometime in the future too...
Cheers 
Paul
@PJ1974:
Let me reassure one thing about a new 50 prime. The 'mid' level 50 prime to replace the ubiquitous 50mm F/1.4 will
undoubtedly have IS based on the 24, 28 and 35 refreshes getting IS. I'm prepared to call that an absolute given at this stage.
Be advised that this 'mid' level, consistent with the other non-L refreshes, will likely cost 2x what the 50mm F/1.4 does now. I've posted a number of times (admittedly, in a minority position) that this increase in cost is (a) consistent with the other refreshes and (b) completely warranted given the additions of IS, USM, internal focusing, wonderful small size and an almost L-series build quality. It's a great disservice to call these refreshes 'budget lenses' -- these are sleeper 'near-L' lenses without weather sealing. Someday, even newer L lenses will put these to shame, but for now, these non-Ls are great options compared the current red-ringed gear.
What we
don't know is whether the 50 w/IS will come with a slower speed / smaller max aperture. So far, that has not been the case. In the case of the other refreshes, the 24, 28 and 35 were all obsoleted by new IS lenses of the
same max aperture -- 2.8, 2.8, and 2 respectively. (Some may think the 28mm F/1.8 is an exception, but the
28mm F/2.8 was obsoleted and the 1.8 is in fact still sold).
But, uniquely with the 50, a super fast lens (F/1.4) is the starting point of the refresh discussion. I have not yet found a reason why 50mm lenses magically can offer such wide apertures so economically while other focal lengths that offer F/1.4 require red rings and a fortune to buy. Perhaps that trend ends when trying to make F/1.4 coexist with IS. Perhaps IS in a 50 F/1.4 makes it prohibitively expensive or prohibitively large/heavy. I'd say based on the other refreshes that the former is possible (as it's never been done*), but the latter is not (the other IS refreshes are quite small).
*Side trivia -- has IS in a lens faster than F/2
ever been done? The Canon 200mm F/2L IS has it, as does this new 35mm. But I am not aware of anything faster than F/2. Just curious.
- A