Last week the SharePoint Designer Team posted an excellent article on the options available to lock particular Designer functionality from certain users.  Quite often I am ask by my customer how to lock down SharePoint Designer to allow users to open a site and create workflows but not allow them to customise the look and feel (master pages).  Now I have a comprehensive article that I can point them to.

http://blogs.msdn.com/sharepointdesigner/archive/2008/11/25/locking-down-sharepoint-designer.aspx