Another similar measure, Libor-OIS, has also been on the increase, although opinion's divided about the market dynamics behind it - read our account.

...Market participants observe that OIS are being used by banks to hedge themselves against central bank policy decisions and major disasters in the market.
“OIS has been one of the most de facto type hedges that you can have on against any position. For example if you want to go out and buy credit, you might want to also buy Libor-OIS spreads too, so if there is a credit event and credit spreads blow out then the chances are Libor-OIS spreads would be blown out too,” said Jim Caron, head of global rates research at Morgan Stanley in New York.
Banks' proprietary trading desks and hedge funds have also been using them to take positions on central bank policy decisions, according to Phillippe Moryoussef, head of euro overnight index average trading at RBS in London...

And here's something else to add to the mix, via Alea: even T-bills aren't considered as safe as they used to be.

The cost of protecting against losses on Treasuries soared to a record on concern that the U.S. government faces higher liabilities with its support for Fannie Mae and Freddie Mac, credit-default swaps show.

Contracts on U.S. government debt increased 2 basis points to 22 basis points at the close of trading in London, according to CMA Datavision, after earlier reaching as high as 24. The 10- year contracts exceeded a previous record of 20 basis points yesterday. Five-year contracts were unchanged at 16 basis points, according to CMA.