The certificate the user is referring to in this case is the signing cert which was used to sign the updatetool.exe file. There is no issue with this cert expiring. It was valid when the code was signed and that is all that is important. If the user were to upgrade to a newer version of Update Center or Glassfish they would get a newer copy of updatetool.exe which was signed by a cert which will expire in about a year.
[Message sent by forum member 'kasso' (kasso_at_sun.com)]
http://forums.java.net/jive/thread.jspa?messageID=387674