I have a Department Mailbox set up for one of my IT groups. Everything
is Outlook 2003SP2 and Exchange 2003. I have two Dist Groups set to
have Editor Permissions to the Calendar, and everything works great. If
I add a single person in addition to those two dist groups, random
people within those two dist lists will be dumbed down the the Default
permission level of Reviewer. This is true of any name added by
himself. I tried one of the user's who can not access the list on
several different machines with different versions of Outlook to the
same result.

Why would adding one person to the calendar break it? That person was
added with Editor permissions also, and is not a member of the dist
groups.