Canon do really fast CD AF on several P&S with Digic IV, AF with a small sensor don't need to be as accurate, but I don't believe Digic IV is the problem with the AF speed. What you need to get better CD AF is higher refresh rates from the sensor, and maybe the main cause for concern for Canon - better control of the lens AF.
USM motors are according to some sources not particulary suited for CD AF algorithms (I've no idea why), but this could be a problem for Canon.
As for new models, a 600D with an articulated screen would make the 60D harder to sell, but would give Canon one unique feature in this segment while they figure out how to make CD AF usable. A 1000D upgrade would make even more sense as just decreasing prices on 450D and 500D (again) wouldn't work too well against the D3100 which feature full HD with AF and a better sensor. A 1100D couold also be the "promised" small dSLR which Canon hope can compete with mirrorless models.
Like you say, DoF must be a big contributing factor to the overall slowness of DSLR CDAF, as I suspect is the size of the lens elements that need to be moved compared to P&S cameras. I don't profess to understand the electronic engineering behind digital signal processing; is the rate at which data can be read off the sensor mainly limited by the processing power of the camera, is it governed by the architecture of the sensor itself, or is it a combination of the two? Certainly, Panasonic seem to have had this problem cracked for a while on their G-series cameras. If Canon is limited by the architecture of the 18MP APS-C sensor, then it was a bit short sighted of them not to consider video when they were designing it!
An interesting point about USM motors, I hadn't seen this before. It goes against my expectations as USM motors are known not only for their speed, but also for their precision. Does this also apply to micro-USM? (N.B. the Powershot SX1 IS had a USM motor.)http://en.wikipedia.org/wiki/Ultrasonic_motor
(O.K. I know it's only Wikipedia!)
An articulated screen would remove one marketing advantage of the 60D over the 600D, but there are still plenty of reasons to buy a 60D and you can be pretty sure that a Nikon D5000 replacement will feature one.
I like you idea that the 1000D replacement might feature the smaller form factor that Canon have previously suggested as a counter to mirrorless. Even though I don't think that simply shrinking a DSLR will be enough to counter mirroless' popularity, it would still be an interesting development.