What's the shortest code to cause a stack overflow?
To commemorate the public launch of Stack Overflow, what's the shortest code to cause a stack overflow? Any language welcome.
ETA: Just to be clear on this question, seeing as I'm an occasional Scheme user: tail-call "recursion" is really iteration, and any solution which can be converted to an iterative solution relatively trivially by a decent compiler won't be counted. :-P
ETA2: I've now selected a “best answer”; see this post for rationale. Thanks to everyone who contributed! :-)
Answer
Read this line, and do what it says twice.