Martin Aspeli recently wrote to the plone-users email list:

It's recently come to my attention that some people are confused about
the role of Zope 3 in relation to Plone 3 and beyond.

The position is summarised here (and has been for a while):

http://plone.org/documentation/faq/zope-3-and-plone

The upshot is that Plone 3 will categorically not run on the Zope 3
application server. To move Plone to be a pure Zope 3 application would
require an almost complete rewrite, and there still technologies we
depend on from the Zope 2 world that have no Zope 3 equivalent (yet).

We are committed to backward compatibility and forward migration paths.
All the core developers have applications deployed on the existing
code-base, and no desire to see every add-on product or customisation
break when moving to Plone 3 (or 3.5 or 4 or 5). We do not want every
existing developer to have to learn a completely new set of technologies.

What is also categorically true is that Zope 3 is becoming more and more
important in our world. Zope 3 can be seen as a library of small
re-usable components. The "Zope 3 application server" (as a direct
competitor to the Zope 2 application server) is but one configuration of
these components. Since Zope 2.8 (and Plone 2.5) these have been usable
inside Zope 2 via an integration component called Five. In a sense, Zope
2 and Zope 3 are converging as some of Zope 2's internals are improved
by borrowing technology from Zope 3. Full convergence is still a long,
long way away, though.

In Plone 3.0, almost every new feature uses Zope 3 development patterns
and components (where appropriate). This is a Good Thing. It has made us
more productive, made Plone development more fun, and made us better
able to re-use technologies that we didn't have to (re-)invent
ourselves. Developers who've yet to learn about Zope 3: we can highly
recommend that you dip your toe in the water - if anything, you will
probably find it fun.

We hope this clears things up for those who were confused or may have
heard erroneously that Plone 3 would run on Zope 3 only. This has never
been in the plan. Plone 3 will be a great improvement over Plone 2.x,
but it is more of a rapid evolution than a complete revolution.

Please do respond to the plone-user mailing list (only) if you have any
questions.

Regards,
Martin and the Framework Team