Various UML Tools support design pattern usage. You typically drag and drop patterns onto your workspace and they appear as UML classes. Its a bit more complicated than this, and raises all sorts of interesting issues. Read the pdf (part 1) and then read the powerpoint and watch the video (part 2).
Here are the slides from a talk entitled "Design Pattern support in UML tools - part II" which I gave in October 2006 to the Melbourne Patterns Group, Australia.
You need flash to view this page propery. Or simply grab the pdf and ppt directly:Design Pattern Automation - Part 1 Design Pattern Automation - Part 2