Recently a client applied SP1 and the December 2011 cumulative update to their SharePoint farm.  Although the installs went without a hitch and the configuration wizard finished successfully they received the 503 Services Unavailable message when attempting to access any SharePoint sites.  A quick look in IIS revealed that all but a few application pools had stopped.  Once we started them back up the entire farm was back to normal (with the added benefit of SP1 and 12/2011 CU of course!).