Getting Python 2.4
First off, make sure you have a version of python 2.4 installed on the system. If you use the one located in the svn collective, it has a few patches that make it work correctly with Lion.
svn co http://svn.plone.org/svn/collective/buildout/python/
Then use that python with your buildout.
Beware of collective.xdv
I didn't have enough time to figure out why, but xdv was making my instance crash on startup with no explanation. I did get plone to startup by upgrading to the latest version of xdv and collective.xdv but it would still crash when rendering a page for me. For now, I've just disable xdv on Lion and at least for a working plone 3 dev machine.
Small post but I just wanted to put it up in case someone else was experiencing the same problems.