Hi,
On fr:, we are doing it now. http://fr.wikipedia.org/wiki/Discussion_Utilisateur:Tipiac/images and the list is getting smaller every minute.
We use the following tags whenever possible: {{GFDL}} {{DomainePublic}} {{FairUse}} {{LicenceInconnue}} (unknown)
So later on, we can either clarify or remove the unknown licence images.
If find this a very good approach and advance.
However, I might suggest to additionally offer at least the Creative Commons license types as further options / choices for a picture publisher (for images only) in WP. Its as easy as with Gnu Fdl to link the license info to the Creative Commons (CC) full text license text of the CC-Website.
I suggest to consider it, because CC use is growing especially for media types of content. And CC should therefore be not left out since more and more picture databases grow under CC which can be used for WP too. And they should not be put under "LicenseInconnue" (rest of the pack) only because there is no other option for the CC licenses available.
Dietrich