In light of the current hostile takeover bid, I think the Xmega has been delayed to maximize cash to address this action.
Nice theory. According to the original press release, xMegas were supposed to be shipping "Q2 or Q3" of 08, and the initial takeover stuff didn't happen till Q4...
Of course Microchip isn't doing great either. Their low-end USB chips were supposed to have been shipping by now, and aren't.