I've noticed vfat filesystems automounted by thunar-volman shows wrong-characters on filenames with non-ascii characters, but then I mounted the same ones with Gigolo (which uses gnome-vfs) and everything looked ok. Maybe this is related to https://lists.ubuntu.com/archives/xubuntu-devel/2008-February/005155.html . xubuntu 10.10 thunar-volman 0.3.80 (Xfce 4.6.2)
It's in exo-mount, not thunar-volman, and anyone can edit /etc/xdg/xfce4/mount.rc to fit one's needs, by adding "utf8=true". (also, it's only for the 0.3.x branch, as exo-mount doesn't exist anymore in exo 0.6)