An official comment:

With the situation at Mt.Gox, we are unable to manage risks in full as the spot price is basically unknown. Margin requirements were temporary increased, so withdrawal requests which would normally be processed are pending now (because otherwise they may not be able to sustain payout requirements of a BTC/USD-4.13 contract due to settle in a couple of days).

We have to care about both sides of the market, and will gladly reduce margin requirements as soon as the market stabilizes (at least opens).