I noticed this with TrueCrypt volumes. Before 10.8, whenever I mounted a .dmg on my iMac, I could access it automatically from my mbp over the network with afp sharing. But after upgrading to Mountain Lion, apparently this is not working anymore.
Furthermore, I if explicitly share the mounted .dmg, I have no error on the iMac , but on the mbp the share is not visible. It's only me or this is a new "feature", may be related to security ?