I know I am coming into this a bit late, and hopefully I can provide some insight into what I have experienced in my sports shooting being an ex Pentax user and a now Canon sports 'tog.
My experience with Pentax AF is very touch and go, my biggest concern with the Pentax was the focus speeds and inconsistency as mentioned especially when tracking fast moving objects such as I do. It wasn't always horrible, but the focus time was really annoying when you would try to capture a split second occurence. I used both Pentax and Sigma lenses for sports shooting, I never had the opportunity to try an SDM lens so I can't speak on behalf of any of those.
As for your prefocussing suggestions, it works if you want still frames but in all honesty we know that in most sports situations there is never a 'designated' path whether it be motorsports, soccer, football or whatever (ok, baseball is an exception).
Now for the dark side (Canon), currently right now I am shooting with a 400D and 40D setup thanks to my girlfriends father who owns one and let me use it as a test-bed. Initial impressions of both bodies are fatastic, ISO performance is very nice, shutter is quiet on the 400D and almost silent on the 40D. Focus speed is most impressive when using USM lenses and is astonishingly quiet (doesn't matter noise wise to me). I shoot with a 70-200 f4 L and have also used the 100-400 L, both of which are stunning glass and rank highly among sports photographers (the 70-200 f4 is the sharpest and highest IQ lens Canon has in its arsenal of available lenses).
AI Servo mode on both bodies works a treat, the 40D sports a nice feature for fast aperture lenses with center weighted focus allowing much faster focussing speeds with a 2.8 or faster lens (for those night shooters). I used a borrowed 75-300 lens initially to shoot Le Mans cars and while the lens itself wasn't acceptable in my books, the USM achieved a solid lock in AI Servo with spot on focus and within a split second. My 70-200 focuses in an eyeblink, as does the 100-400 (another perk to the L series glass) and I've yet to hear any complaints about either lenses. IQ is astounding on both lenses, even with a 1.4x TC on my 70-200 f4 it holds up amazingly well with a very minimal loss of IQ and hardly any noticeable focus speed drops.
Overall, the 40D is a cool body with lots of great features however if you are looking to stretch your money to the absolute maximum I would suggest going with a 30D (they can be had up here for $999.99 with the 18-55 kit lens) and dropping the rest of the money into your lenses. A 100-400 on the grey market will set you back roughly $1100, a 70-200 f4 about $500, 70-200 f4 IS $700-800. Another worthy opponent in the Canon field is the Sigma 100-300 f4 EX lens (much like what you already own), an extremely solid contender and tack sharp with superb IQ (Sigma stuff I tend to bump my settings +1 with though).
Both systems are great, I still love the IQ I got from my Pentax lenses but the decision for me was simply made because I was taking my motorsports photography more seriously and needed equipment that could keep up with the demands I had. Hopefully some of the information provided was useful, if you need any tests of sorts or want some test shots feel free to let me know and I will snap some for you