I have not upgraded to Google Earth 5.0, so I can't speak to this personally. But there is a thread in my Google Certified Teachers Group that looks like it might be valuable to school IT folks.
Seems that upgrading to Google Earth 5.0 has caused lots of issues in school labs. I've attached a PDF of the thread as of this morning. Seems that tht upgrade problems did not allow a school to go back to previous versions of Google Earth.
Just a heads up in case someone was thinking of doing the upgrade.