Windows Vista SP1 arrived in March 2008 and Windows XP SP3 in May 2008. In both cases, the service packs went out via Automatic Updates a month later.
Vista SP1 will no longer be blocked after April 28, 2009 and Windows XP SP3 will no longer be blocked after May 19, 2009. The service packs will be automatically offered to the user if they have AU turned on, though he or she will still have to agree to install on their machine. If AU is off, the service pack will not be offered until the user hits "Check for Updates."