"This causes a severe regression for programs that need to identify the Sun/Oracle HostSpot VM such that they know whether the "-XX:MaxPermSize" argument needs to be used or not."
Basically, the eclipse.exe launcher checks if the 'Sun Microsystems' string is present in order to add the -XX:MaxPermSize VM parameter. On Monday, Oracle provided a new version (build 07) of Java 6 Update 21, reverting the vendor name back to Sun Microsystems Inc for the moment. So, if you haven't used the auto-update yet, you'll have nothing to worry about thanks to Oracle's fast reaction.
Oracle will be changing the vendor name in Java 7, and there's already a patch available to deal with this change when it comes about.