Users & passwords in shared /home partition
 

I'm thinking of putting a second distro onto my hardrive so that I can play about without knobbling my Slack 10 setup. I have a separate /home partition which I could share between the two distros. My question is, would I have to use the same user names and passwords in both distros for this to work?

You can chose any passwords freely. Using same usernames is not strictly necessary, but makes a lot of sense. Same userIDs is a good thing, or else the permissions and ownerships of the files doesn't match between the systems.

Easy way to archieve this is to copy /etc/passwd and /etc/group (or critical lines from there) to your new system. To get passwords too, copy also /etc/shadow.