I tried this too, using a Samba share on an XP box kicking around the house. I was able to save simple files (say, a blank canvas with a rectangle), but more complex ones (a copy of the Mind Map stencil) got an error. Copying the stencil into the simple file made it no longer work. I'll mail a copy of the file to
omnigraffle@omnigroup.com as it seems to be too large to attach to this posting. Seems like it ought to be easy enough to reproduce!