According to Microsoft's MSDN Technet blog, this update was still in beta stage and wasn't supposed to be released. It was all a terrible mistake, you see.
Some (lucky?) people who managed to grab it and installed it report (MacFixit) that it has a number of bugs. For exemple, Entourage will crash everytime you try to connect to an Exchange server over SSL.
The best? The aforementioned blog post advises you to:
We recommend that anyone who may have installed these pre-release updates to uninstall [sic] them.Yeah really? How???
