I was working on a Dev environment today and found that when the developer had configured SQL they left themselves as the only sysadmin. This was before we had created a standard Dev image and I found myself unable to log into SQL.
Apparently in Server 2008 you no longer get access to SQL by default if you are in the local admin group.
Fortunately I found this script @ http://archive.msdn.microsoft.com/addselftosqlsysadmin/ which did the trick for me. Kind of a useful little item to keep handy.