Paul Graham wrote about The Python Paradox. Unlike his earlier talk, this one actually had a good point.
A person who goes out of his or her way to learn a non-mainstream language is obviously interested in programming, not just doing it for a job. While Paul's focus on Python may be a bit obsessive, it is certainly true that multi-lingual developers tend to be better.
One of the better tips from the classic "Pragmatic Programmer's" book is to learn a new language every year or so, preferably one from a different paradigm. The exposure to new ideas and concepts helps you grow and develop as a developer.
