An update to SQL Server 2008 Books Online has been published to TechNet and MSDN. It will show up on Microsoft Update within a few weeks. You can see the list of new and updated topics here:
And if you don't want to wait for Windows Update, you can download an installer to update your local copy here:
The MSI file is 144 MB. When told that a lower version has been detected and asked if you want to update your existing version, say yes. I can't think of any reason why you'd want to keep two different versions of SQL Server 2008 Books Online side-by-side. Unlike a few previous BOL updates, the actual documentation date agrees with the download page and file name (the documentation seems to have been checked in January 23rd). The download page says it was published January 30th, but I don't think the page was actually made live until February. Which begs the question, why wouldn't they label the download and actual content as a February update? Seems like it keeps it that much "newer" (remember when they released Windows 95 in October of that year, and then by the following March people were already asking where is Windows 96?).