Originally, you had to forge the knife that you used in the competition. The competitions grew out of the ABS smith tests - where you have to prove you made a good knife by using it in various prescribed ways (chop, hanging rope cut, etc.). So you had to be a smith (no stock removal) and make the knife you used in competition.
A few years ago, there was a move to open things up and have different divisions of competition - including one that would allow people to use production knives. The ideas was to make it more of a sport, with sponsors, specialized competitors, and of course to attract non-smiths.
I think Browning was the first to release a production competition knife. Spyderco had one on the drawing board, and was an early supporter of the broader competitions.
(the above info is from memory, so no links to the cutting comp websites, rules, etc. Take with a grain of salt and fire up Google for confirmation)
Which begs another thread - which Busse pattern would make the best competition knife, and how would you modify it? (different grind, stock thickness, etc) Without looking at the rules, the SH-E comes to mind. Maybe a zero-tolerance version with optimized edge geometry.