@DoctorOhh: If anything, I would expect the reverse, i.e. the new backend actually releases the lock on metadata.db more frequently than the old one did.
However, let me take this opportunity to remind everyone that having dropbox syncing your library at the same time as calibre is using it is a very bad idea on windows. On windows *any* file access by any program causes the file to be locked. Therefore, if dropbox is reading or writing to *any* file in the calibre library, calibre will not be able to move/write/delete that file. This can lead to all sorts of failures in calibre -- conversion jobs failing, changing title and author metadata failing, deletes failing, not to mention library corruption.
On windows you should never run two programs that access the same calibre library at the same time. Pause dropbox syncing when you start calibre and resume it after you quit.