According to information just released by an admin from the Sega of America forums, Virtua Tennis 2009 will be delayed one month and now will be shipping in early June. Although there has been no official word on the delay (and the official website still shows May 2009), the admin mentions Sega is probably polishing off the Wii version.
The Wii version being completed pushes back the PS3, Xbox360, and PC versions as well. Hopefully the delay allows for more content to be added and better online experience (for PS3/360).
N4G Note: Interestingly, the Wii version will now compete with Tiger Woods 2010 during that release timeframe, and both are sports titles featuring Wii MotionPlus.