Very painful. I learned this week that patching Solaris and Windows O/S for DST 2007 changes is inadequate for some Oracle and Microsoft applications. For example: depending on the Oracle version, subsequent Oracle patches may be needed for the Oracle Rdbms software and Oracle JVM (optionally installed) in each database.
Moreover, I understand that Microsoft apps have patching requirements that go beyond the O/S patching. The specific example I heard about this week is that Outlook versions prior to Outlook 2007 will need application patching else existing appointments will be an hour off come March 12th when the new DST rules go into effect (even if you patch the MS O/S for DST changes).
So, this is just a off-hand sampling of potential issues I'm aware of. Kudos to kool delhite for bringing up this topic in a perl context.