When is bloons tower defense 4 coming out

This game is now publicly available and can be played here: http://www.keepbusy.net/play.php?id=bloons-tower-defense-4

On 26. Oct. 2009

This date is confirmed by the makers of the game at:
http://ninjakiwi.com/blog/bloons-tower-defense-4-gameplay-vids-and-release-date