Don't be fooled by the poor title on the page, which still says "SQL Server 2005 Books Online (September 2007)" (clearly a leftover from an earlier BOL update). The text under the overview section also mentions the September 2007 update.  And the page thanking you for downloading says:

Thank You for Downloading
SQL Server 2005 Books Online (September 2007)

The file to download is definitely SqlServer2K5_BOL_Dec2008.msi so this really isn't the September 2007 update.  Curious, though, when I install the update my Books Online document explorer title becomes "Books Online – SQL Server 2005 Books Online (November 2008) …"

It would be great if the publishing process on the downloads site were less prone to these errors and inconsistencies.  Most people will be fine, because they will just know that they have the latest version, in spite of the stuff around.  But I think a lot of people will not discover the download because the title still says September 2007 (an easy fix, let's hope).  And I think there will be other confusion when colleagues ask, "did you get the December BOL update?" "No, I only have the November, is there a newer one?"

https://www.microsoft.com/downloads/details.aspx?FamilyID=be6a2c5d-00df-4220-b133-29c1e0b6585f

What has changed in BOL, you might ask?  Or in other words, "should I bother downloading it?"  Well, there is a new topic called "What's New in SQL Server 2005 SP3," which will be of interest to any^H^H^Hall of you interested in applying this update when it is released – probably also available online, but as others have discussed before, I want that stuff local also, even if it means I wait longer for updates.  And the following sections had topic updates:

• Installation (1 update)
• Database Engine (9 updates)
• Analysis Services (1 update)
• Integration Services (3 updates) 
• Replication (1 update)
• Reporting Services (8 new topics, 9 updates)
• Language Reference (15 updates)
• Tutorials, Samples, and Sample Databases (2 updates)
• Tools and Utilities Reference (1 update)
• Error Messages (9 new topics)

So, if you like having Books Online local, there is no reason you should put off downloading and installing this update wherever you use it.