Problem solved--I don't know if it has happened to other people or not, so here's what I found. For some reason, the passwords used by Samba on the Macs were corrupt--when I tried to add a user in the Sharing panel, or remove his Samba access, I was consistently told that the password I entered was incorrect, on both machines, two accounts on each. After going to the Accounts pref pane and resetting the passwords for these accounts, the new passwords were accepted by the Sharing panel. They were also now accepted when I connected to the Macs from the Linux VM(The MacBook is new and I migrated my user settings over from the G5, which might explain why they were both broken the same way.)