The Zope development team decided "Zope 3.1 is more than ready for production use". The next zope upgrade will have consequences for users running Zope 2 applications.
Zope 3 is a complete rewrite and a very different thing than Zope 2. It will sadly break many existing applications available for the Zope 2 platform. Mainly, a complete enterprise CMS is not yet available for Zope 3, so there is no substitute for CPS or Plone yet. If you run one of the above-named products, you must use Zope 2.
Upgrading zope-2.x to zope-3.x on archlinux will break your Zope 2 instances. Hence you have to install the legacy package zope2 which is also available in the extra repository. You can install Zope 2 and Zope 3 simultaneously, there are no file systems conflicts.