I'm not so sure that this data is correct, does it really move that fast?
BTC/USD Long Position
We charge you USD Sell interest, and pay you BTC Buy interest every hour.
BTC/USD Short Position
We charge you BTC Sell interest, and pay you USD Buy interest every hour.
And BTC sell interest dove an hour or so ago, thus the positive swap rate.
Check out the interest rate chart on bitcoinica (http://charts.bitcoinica.com/interest_rates
), the dips are where the short swap went positive, this dip does seem to have lasted longer than the last few though.
Edit: and as I predicted it just flipped back. I may start a subscription service for swap rates....