I'm running Zope and Plone on FreeBSD, both on i386 and alpha platforms.
I seem to have problems no one else experiences (like PloneExFile being
unable to handle attached files over 25 MB, or LDAPUserFolder causing
the entire Zope instance to get stuck in a loop). Like MediaWiki,
Apache, and PHP, most of the Zope and Plone developers do their
development on Linux. Unless you are prepared to deal with the bugs,
you probably should start with Linux, too.