Today is the last day for the Fed's open market operations known as "QE2", so Gross apparently took this opportunity to re-emphasize his current position that no one will buy Treasury bonds once the Fed stops.
Here's a link to his "tweet" and I quote: "Who will buy them now?" (Ed.: Unbelievable!)
I think bonds are actually going to rally once the Fed exits this marketplace, as they have been acting as a deep-pocketed scale-down buyer, in a market where there has been much speculative selling.