In SP2010, is customising workflows using SharePoint Designer configuration or customisation?
I believe it is customisation.
The reason for this is because you "Configure" the Workflow you are using a gui programming language of sorts in order to give a different flow.
It requires research planning and development time. Anything that uses these resources is customisation.
I believe its neither customization nor configuration when you create a Workflow in SharePoint Designer. Its an additional feature that comes out of the box, that you are using to make your SharePoint lists or libraries extensible.
Customization basically refers to custom code or webpart that you develop in SharePoint and configuration should refer to any config settings that you do like in the Central Administration.