If it is possible, it must only be possible for the Dumbledore or Voldemort-level magically powerful - I don't think we'd (logically speaking, not that this is JKR's forte) be able to avoid hearing about it if any sixth year could get the hang of it. As far as the average witch or wizard would be concerned, it's effectively impossible. That would leave a loophole if anyone wanted to use it - Voldemort Apparating ridiculously far distances might not look as impressive onscreen as swooshing around like a bat, but it probably would have been a more impressive magical skill for him to have. Although since any initially "really hard" magic in the series gets downgraded to the level Harry can teach to other kids, maybe it's just as well - it would have thrown a wrench in so many things if suddenly the whole DA could do it (and would be prone to overuse by Sues, no doubt).