Someone once told me (so it must be true) that in a freestyle event swimmers can use whatever stroke they like (as the name suggests)... within reason I guess, no snorkels and flippers (sorry simonp)... nor heat stroke, or two stroke, or Basingstroke (ok, I'm reaching now)...
..anyways, as people are inclined to use 'front crawl' (is there a back crawl?) as it is generally the fastest stroke, the 'freestyle' name has become synonymous with it. Apparently, strictly speaking, one can use butterfly (or back stroke, or perhaps even doggie paddle) in a freestyle event. You probably wouldn't win though. There's hope for me then. I'd give the 100 metre dog paddle a go.
Oh, and Wiki confirms so it really IS true.