I have had the MkIII for a while now and used it extensively for sports and air shows. Never had a problem with auto focus or at least from what I can tell. I also do not make a living using my gear either, but consider myself a serious amateur that knows how to use his gear. I find it funny how the internet has turned these "issues" into such a firestorm.
While it may be discerning that Canon has a new update for the MKIV, it will not cause me to cancel my order for it. People seem to forget how complicated a device digital cameras are now a days compared to film based cameras. It is also no excuse for Canon to rush out gear without first testing it thoroughly. However, I also have not heard of any reports about AF performance issues with the MKIV that have been substantiated. So I will wait and see what the verdict is on this firmware release, as it may well be just a performance enhancement.
On a side note. I was able to use the MKIV a couple of weekends ago and it is going to be an excellent camera. The AF is will take a bit to adjust to and the movie mode is unbelievable. Not crazy about the extra pixels, but it is a nice added bonus.