GuamPDN.com ("Guam's complete source!") has an article up by Duane George, who tells his story of woe: he got banned from the game for suspected Arena win trading, and had to deal with 72 hours without the game. Blizzard, obviously, doesn't provide any information on how many players get banned from the game, and it would be even harder to determine the number of false positives out there like Duane: people who didn't do anything wrong but end up getting banned anyway. We've heard stories here of course, but this is a tough area to investigate by its very nature.
Fortunately, this wasn't a permanent ban, and while he did apparently lose some Arena rating and the gear that came with it, his character wasn't too much the worse for wear. A 72-hour ban isn't too big a deal, so Blizzard probably hands those out with much less consideration than a permanent ban anyway. But we're sure Duane isn't the only case out there -- as small as the number may be, there's almost definitely other players like him, banned for doing nothing wrong at all.