Comment 41 for bug 31471

Another workaround that can fit some needs : use the package autofs
You can configure it so that it mounts your network shares in a location like /mnt/autofs/servername/share
You can then put a bookmark on this directory, and it seems to be always displayed in open/save dialogs (tested on Firefox and Thunderbird)

Autofs mounts and unmounts the shares automatically when they are used/unused
If you need to use credentials (login/password) to connect to your SMB share, it's less straightforward but seems to work with a bit more config. See http://doc.ubuntu-fr.org/autofs_avec_samba#acces_aux_partages_avec_authentification (in French)