I believe until SQL Server 2000 (or even SQL 2005) it was easy to remember the release version of your SQL Server instance.
Since the changes affected from SQL Server Release Services and inception of Cumulative Update packages it is hard to keep a track of list of builds and version releases. Let it be a controlled installation of service packs and cumulative update packages, you must know the information in order to check what is fixed in the release.In general the best practice is to apply only service pack releases patch to the active instances of SQL in your data platform, but it is essential as a DBA to keep a track of cumulative update package releases. The incremental changes and naming convention of each cumulative package is required to understand whether it is important for your environment or not.
For the ogransiations where they do not have a 'DBA' then its better to follow few links down in the blog to ensure they are following best practices (at the least)!!
The first and foremost helpful is 321185 How to identify your SQL Server version and edition link, then you must obtain the information Incremental Servicing Model for SQL Server along with the naming schema for SQL Server updates and software update terminology that is used to describe the software updates.
Coming back to keeping track of build version and cumulative update package releases along with service pack versions, look at the SQL Server version database link which is my favourite and always follow to keep up the release information. Recently got into this MSDN blog SQL Server history & sqlserverbuilds.blogspot.com/ blog too which highlights the fixes that are required. However the information from SQL Server version database is much broader than these 2 links which will go back to stone-age versions of SQL Server too.
Further more information on taking good help of Slipstream from SQL Server 2008 & SQL Server 2008 R2 versions refer to Microsoft SQL Server 2008 R2 Administration cookbook which is a recipe based book that will help you to add the required features, how to do it and how it works scenario.
Hope this helps.