Well, let me be more precise: there is an updated version of Books Online online.  As reported earlier by Buck Woody, the online version of SQL Server 2008 Books Online has been updated (we are seeing the updates now but the content was refreshed June 29th).  The download has not been made available as of yet; this usually takes several days to get the packages together, get them up on the download servers, and then distribute the links throughout the download sites and other Microsoft properties.

When I was looking around to try and spot the downloadable version, I was quickly re-introduced to some peculiarities I've noticed before but didn't care enough to comment on.  Most importantly:

• the front page of the online version does not give any indication that there is a downloadable version, never mind provide a link to it; and,
• the main SQL Server 2008 page (http://msdn.microsoft.com/en-us/sqlserver/bb671064.aspx) points to a download page (http://msdn.microsoft.com/en-us/sqlserver/cc514207.aspx) which says you are downloading the January 2009 update.  Of course the download page behind the links actually offers the May update.

I complained about this more formally here:

#472985 : DOC : Make Books Online downloads more discoverable and current

Anyway, you can watch this page to see if they update it when the new version becomes available for download (or, as Buck suggests, you can wait for Microsoft Update to download the new version for you).