I use WebFaction for my (one) Plone project. Customer service and uptime are both good. They support Buildout -- in the sense that they support pretty much anything Python. When you first sign up they will set up a working Zope/Plone instance for you if you ask them to -- this will not be Buildout-based, however. To do the Buildout thing you have to ssh in and do it yourself. Which is what most pros would do anyway -- just thought I might clarify, since I have recent first-hand experience.