Fire up your SQL Server 2008 Books Online (BOL) and check the date. You should find this in any article (except for the very start page).
RTM of SQL Server 2008 shipped with a BOL which has ... hmm,... doesn't have a date (just checked in a virtual machine). OK, so if you don't find a date (year month) you have the BOL which shipped with RTM.
The first BOL update was released 2008-09-30. You will see this as (August 2008). This was made available on the web as well as a download (http://www.microsoft.com/downloads/details.aspx?FamilyID=765433f7-0983-4d7a-b628-0a98145bcb97&DisplayLang=en). This BOL did not list what was changed from prior release since this was considered RTM of BOL and the BOL that shipped with RTM was considered RC0 (which explains why this BOL lists Aug 2008 when it was released late Sept 2008).
Today I learned that MS has released yet another release of Books Online. This is only available on the web, and you see this as (October 2008). Now we do have listing of what section are new and what were expended on. I admit that I prefer locally installed BOL, but I guess that MS has resource limitations the same was as all of us do. Check it out at http://msdn.microsoft.com/en-us/library/dd239407.aspx.