SP2010: Associating a Visual Studio workflow with a content type
In SharePoint 2007, when you created a workflow in Visual Studio, it allowed you to associate it with a list or a content type by default. In Visual Studio 2010 however, it only allows List association by default.
To allow your workflow to be associated with a content type, you need to modify the elements.xml and specify both list and content type as follows:
See http://msdn.microsoft.com/en-us/library/aa543430.aspx for more details.