Someone has revealed/leaked the card Oubliette in Double Masters, with the errata'd text "When ~ enters the battlefield, target creature phases out until ~ leaves the battlefield. Tap that creature as it phases in this way."

I am pretty sure we are going to see a change to phasing rules, because I am nearly positive that "phases out until..." is not currently well-defined. The phasing rules don't mention it, and 610.3 currently only addresses zone changes.