The Qt version hasn't changed between 0.6.55 and 0.7.x. What has changed (I think) is that earlier calibre was not using QUrl.fromLocalFile it was directly passing a filename to QDesktopServices. While this used to work for UNC paths, it broke in several other more common cases, so I switched it to QUrl.fromLocalFile
If you can figure out the correct way to pass UNC filenames to QUrl I'll be happy to make the change.
You can test by doing
Code:
calibre-debug -c "from PyQt4.Qt import *; app = QApplication([]); f = r'\\server\whatever'; url = QUrl.fromLocalFile(f); QDesptopServices.open(url)"