it worked in 12.04.
It doesn't work anymore since my migration to 12.10 last week.
Here is the fstab line :
nas:/volume1/dropbox /home/guillaume/NasDropbox nfs defaults,user,nolock,nfsvers=3,exec 0 0
and here is the result:
-bash: ./unison_photos_gentoo.sh: Permission non accordée
guillaume@htpc-arctic:~/NasDropbox/Dropbox/scripts/unison$ l unison_photos_gentoo.sh
-rwxrwxr-x 1 guillaume guillaume 71 May 11 2011 unison_photos_gentoo.sh
which in English is something like permission denied.
In 12.04 I added exec in the fstab line to make it happen.