A key learning for everyone trying to upgrade 2007 custom solutions to SharePoint 2010.

If you have custom content types which inherit form the Page publishing content type (or any publishing content type), you will be able to upgrade your sites and create subsites - BUT you will not be able to create new site collections, or activate your custom content type feature on a standard publishing site, UNLESS you have the Inherits="TRUE" added to the ContentType element.

It seems not be a hard requirement for non-publishing content types.

Bottomline is that the Inherits="TRUE" attribute is new to SharePoint 2010, and when using inheritance you must make sure always to include this for new custom content types.