Confusing! First the PlayStation only supports one PlayStation Network account per handheld, then it doesn't. Now, it does again. What?
Sony told Wired that, contrary to reports that were contrary to reports, the PlayStation Vita will indeed only permit one PlayStation Network account per unit. If you want to change accounts you'll have to reset the Vita to its factory settings and delete everything off the thing.
So we're back where we started, and where we started sucked for those with multiple PSN accounts. Disappointing!