I see you can get long on cex.io, but this isn't really a GH/s "futures" contract, rather physical delivery, which makes it hard to short (i.e. you need to mine to get the GH/s to sell).
Looks like OTC will be the only option, but perhaps someone knows better...?
The Deprived Mining Speculation (DMS) set of securities (https://bitcointalksearch.org/topic/btc-tc-deprived-mining-speculation-dms-228327) has a mechanism that allows shorting of mining-contracts. Currently it's not trading due to the BTCT shutdown, but latest word is that it is due to be relisted on a brand new exchange. Needless to say, shorting mining with this instrument has been very profitable since the launch back in june.