If you want to see what the service pack is going to touch or replace, if you do a search on the service pack on google, and go to the Microsoft page to download that service pack, there should be a link to a technet article that will tell you exactally what the service pack is fixing, and the DLL's being replaced.
When you look in SSMS you will see the version number of SQL Server(2005 & 2008).
9.0.2047 is SQL 2005 Service Pack 1
9.0.3042 is SQL 2005 Service Pack 2
10.0.2531 is SQL 2008 Service Pack 1
There are a multitude of build numbers out there these are just 3 different examples.
The Dll's the build of the SQL Software is updated and changed just like the other folks said. It is not something you can move from server to server.
You could take Ghosts images or Tape Backups or some other image backup of your server's drives before an upgrade, in order to revert back to your previous installation should you encounter a critical failure after performing a Service Pack upgrade.
But you would only restore those backups if a critical failure occured to your system. DLL's, and system dll's would not be enough, you would also need the registry and possibly COM+ services to be restored in order to revert back to an previous service pack version of SQL.