The "Design Pattern Automation Toolkit" -- an open-source tool, on SourceForge:

http://dpatoolkit.sourceforge.net/

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As seen on the Gang-of-Four DesignPatterns discussion list:
http://groups.yahoo.com/group/gof-patterns/message/48

Requires .Net

It seems to support C# and VB.NET at this time.
There's mention of Java support, but it's not clear if it can recognize or generate Java at this time.

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''Hmmmm...  Interesting, but I'm not sure I'm impressed.  Like, what does it do for me?  -- JeffGrigg''

Hey jeff, it allows u to use design patterns in ur design, when u can view the class diagrams , generates code stubbs (in java too!! dear wiki), and help for reverse engineering for .net aware languages!!

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I can see how a tool like this might make it considerably '''harder''' for me to effectively utilize design patterns in my code. For a pattern to be most useful, it should emerge from the needs of the design, right? A tool like this encourages me to think hammer-first: "Well, I need a Composite pattern here. Let's get one from the toolkit! Now, how to shove this into the rest of the code...."

Other aspects of this project seem interesting, if a bit ambitious. If it can data-mine a pattern out of a snarl of old code, it might give me an interesting perspective, perhaps a direction in which to move. But code generation would not sit so well with me. 

Refactorings are basically mechanical. Design is much less so. Give me a power screwdriver (or a RefactoringBrowser), but not a blueprint-generator. -- PhilGroce