If you only know one language, no matter how well you know it, you're not a great programmer.
There seems to be an attitude that says once you're really good at C# or Java or whatever other language you started out learning then that's all you need. I don't believe it- every language I have ever learned has taught me something new about programming that I have been able to bring back into my work with all the others. I think that anyone who restricts themselves to one language will never be as good as they could be.
It also indicates to me a certain lack of inquistiveness and willingness to experiment that doesn't necessarily tally with the qualities I would expect to find in a really good programmer.
