I get asked quite a bit about auditing in SQL Server. By "audit", people mean everything from tracking logins to finding out exactly who ran a particular SELECT statement.
In the really early versions of SQL Server, we didn't have a great story for very granular audits, so lots of workarounds were suggested. As time progressed, more and more audit capabilities were added to the product, and in typical database platform fashion, as we added a feature we didn't often take the others away. So now, instead of not having an option to audit actions by users, you might face the opposite problem - too many ways to audit! You can read more about the options you have for tracking users here: http://msdn.microsoft.com/en-us/library/cc280526(v=SQL.100).aspx
In SQL Server 2008, we introduced SQL Server Audit, which uses Extended Events to really get a simple way to implement high-level or granular auditing. You can read more about that here: http://msdn.microsoft.com/en-us/library/dd392015.aspx
As with any feature, you should understand what your needs are first. Auditing isn't "free" in the performance sense, so you need to make sure you're only auditing what you need to.