I can see a lot of people have been downloading the plugin. There have been daily changes being made to this so you may want to stay current on the latest version.
http://roundcubegoogle.sourceforge.net/ [1]
As of rc svn version 4804, no modifications to RoundCube core are necessary
The latest version just uploaded no longer requires change to the Zend code.
Drag and drop support is now fully functional.
Only one problem left and anyone able to offer suggestions please email me at rc [AT] deltatechnicalservices.com
The one known remaining problem is that if you attempt to drag and drop a large number of contacts to or from the google_contacts to another database, it can take some time and there is no notification on screen to let you know that is in progress.. Just wait until you see the notice on top saying it is complete. The same is true if you attempt to delete a large number at one time like 30 or more.
So download, and enjoy and please let me know if you have any problems or questions.
Links:
[1] http://roundcubegoogle.sourceforge.net/
List info: http://lists.roundcube.net/dev/ BT/aba52c80