I just wrote a little Python script that imports the vocabulary data from Paul Hemetsberger's
dict.cc project into Leopard's Dictionary.app.
This is great if you need this fabulous online dictionary while on the move.
Update: A new and much easier installable version can be found here.
The code is quick 'n dirty and some strings aren't processed correctly. This is compensated by the links to OS X's build-in New Oxford American Dictionary.
Regrettably dict.cc's license prohibits redistributing the vocabulary database. So, if you want to use Dictionary.app to translate words from german to english you'll have to build it yourself: