Hey, if you don't mind, i'd like to post an additional question I have on this subject.
I was shooting with a Canon 50D and also the 55-250mm IS lens
This weekend I went out and took some pictures at a Karate Tournament. I tried shooting with AF Servo as I thought this would follow to subject through the different AF points.
So I started using only the middle AF point, pointed it at my subject's face, and then just kept my trigger pushed half the way down, while i would compose so that not only my target would be in the picture, but also his opponent...
The problem is I don't really think it worked

is it because all the fighters were wairing a white outfit and the camera goes from one guy to the next? Or does the AF lose its target after a while?
The problem was I couldn't just keep my target on the middle AF point, as then I wouldn't have his opponent in the picture, but just his legs or something... and I tried choosing a different AF point (the left/right ones) but I didn't like the way it handled that...
What do you guys think I should do in situations like that?
(or is the picture in focus but is it cause of the ISO noise cause it was not ideal light, hard of course to answer this as you can't see the picture, i'll post asap)
I'm going to post it soon, but as I'm at work, I can't access the pictures and post them, and I don't think I'll have time to do it in the next couple of days, but I'm so anxious to see what you guys have to say about the whole AF Servo etc, that I would just post my question here, already, while I wait to start my own topic, incl my pictures.