There were rumblings about this happening but it was officially announced today: http://twitter.com/Yankees/status/340174061290278912
I love this idea and think it will be a great day for Hideki Matsui and for Yankee fans. Plus, I have tickets for that game so I am really looking forward to seeing him and getting one of these bobblehead dolls.
After he announced his retirement over the winter, I wrote a retrospective about my favorite Matsui moments in pinstripes. You can read it here. Feel free to add your memories in the comments.