I try not to post things that others have already discussed, but I make a few exceptions. I'll (re)tell you about security issues and service packs, and I'll also mention when Books Online is updated. I run into so many issues that are already covered in BOL, so I think it's important to track when new content is added or altered.
You can find the On-Line version (which is always the latest) here: http://msdn.microsoft.com/en-us/library/ms130214.aspx
You can find what has changed for this version here: http://msdn.microsoft.com/en-us/library/dd776386.aspx
You'll get the latest version in your Microsoft Update Services for the installed BOL. How can you tell what version of BOL you have installed? Look in the title bar.