It's now available. You can short with a $40K margin requirement! You only need a 9K margin to go long. And it's $5/contract.
And don't forget that you need special cryptocurrency trading privileges to trade the bitcoin futures. It can take up to 24 hours for approval. I also have an account with Tradestation and they don't require any special permissions.
Too bad IB isn't gonna make much money off this since the current volumes are outrageously low. Been looking at some daytrades, but the spreads are way too high. Maybe the CME will bring in more traders next week?