While it would mean an easy transition, I'm not sure this is the best way either. As others have noted, I've seen odd cases of things performing extremely poorly or failing in strange ways under OpenJDK 6.

Remember, if people have Sun's Java installed now, it's because they deliberately opted to install that over OpenJDK - Sun's has always been in the partner repo, which is disabled by default, OpenJDK being the default on Ubuntu. Silently moving people back to OpenJDK would cause all kinds of non-obvious breakage.