We also know a content type is scoped at a site collection level, does that mean reusable workflow can be used cross all site collection? Yes, but you need to "Publish Globally" (in SharePoint Designer ribbon) to make it be reusable in a whole site collection.
SharePoint 2010 has a new feature called Enterprise Content Type which allow content type to be reused beyond a site collection scope . See this article on Technet for a completed guidance on how to setup a ECT. If the content type is published from content type hub and subscribed by another site collection or even another web application, The content type can be reused in a farm scope. The question is, can its associated workflow be reused? The answer is No at this time.
To summary up:
