John Tallman has a clarification on the play:

I was at the Yankees game last night sitting in a good enough spot to see the Ensberg/Matsui mish mash. Matsui definitely stayed within the baseline rule when he avoided Ensberg. From my vantage point you could see the play perfectly. The confusion was understandable because the way Matsui jumped made it look like he went outside the baselines, but he actually didn't move that far, maybe a foot at the most. If Ensberg had tried to tag Matsui higher on his body he would have been out, he just managed to move his big butt fast enough to avoid it.

Now if only the broadcast team could actually get a good vantage point on the play, even with the umpteen different angles and the closeup on Matsui's butt.