SUS is being "phased out" in favor of the new "WUS" - Windows Update Services. WUS will be superior to SUS in the fact that it will allow for the updating of much more than Windows itself, including things like Office, Exchange, etc.
Furthermore, it will also allow non-critical updates for Windows to be distributed through WUS. It will use a SQL server for logging (either MS SQL Server or MSDE).
You can find more information on it
here