"Shorting" turns out to be a complicated construct that needs a lot of regulatory pieces in place.

The exchanges are largely unregulated, and have a track record of messing people around when a price changes quickly.

Bitfinex is big on offering complicated margin trades that nobody else does ... but you can't get actual money out at the moment.

You can margin trade on exchanges like Poloniex right now. Whether or not it's a bright idea is another matter. Even if you assume that it's presently overvalued, how can you know when (or even if) the bubble will pop?

Well that's always the risk with shorting I suppose, I just meant as safe as shorting can reasonably be :)

Thanks for the Poloniex tip though, looking into it now!