WordPress 3.0 is the 13th release of the script with major improvement. The codename of this version is “Thelonious” A total of 218 contributors involved in this release. Major feature of this release include new sexy default theme called Twenty Ten.
Also MU and WordPress have been merge together for person that have multiple blog before can use single wordpress for all the blog.
The new version also fix 1217 bug and have features such as bulk upgrade.
More info please refer to the wordpress blog.