In the comments to Jeremy’s post, Marten Mickos tries to clarify that it is more specifically high-end enterprise type add-ons to the core MySQL system that will be only available to MySQL Enterprise and that the license for this is as yet undecided.

There’s no real way to effectively close source an open-sourced application, but developing new features in a closed source edition is very doable. It remains to be seen whether it will be a successful attempt as MySQL has gained much from it’s Community use in terms of both release testing (the shear number of MySQL users out there) and goodwill.