Update 2 for Civil 3D 2011 was released on June 30, 2011. You can find the download page here, along with the various language README files, which include a list of items addressed.
I was hoping this update fixed one particular bug, the one that causes the Prospector and Settings tab to go away after you run the ._PREVIEW command - but it does not. This is still an issue even after installing the update.
One thing to note: You must apply Update 1 before you can apply Update 2.
Lastly, to keep things interesting for us, after Update 2 is applied, the software will report that is is currently on "Version 3".