I\'ve been researching a solution to this all week and while there have been solutions to similar problems there are none that address and rectify this problem directly.
I just installed Eclipse + GPE4.2 in a new machine using GAE SDK1.7.5 on Juno Service Release 1 Build id: 20121004-1855 and was forced to copy the JDBC driver at appengine-java-sdk-x.x.x/lib/impl. Initially I had forgotten about this and I spent about 2 days debugging my code.
It would be nice if there was more information available on GPE page issues. It would be nice to be able to Google more information.