Much as I hate to post the obligatory Joel blog post, I agree with him here. C is the lingua franca of programming. I can't think of any high-level language that can't interface with it somehow. Because of that, C is still a popular choice for systems programming type stuff. You simply can't interface with some OS-level things without C.

Besides that, what do you plan to do when your high-level language isn't fast enough? Knowing how to write C is especially important when you're using a high-level, dynamically typed language like Ruby, Python, or PHP. But even Java and C# programmers need to drop down to C from time to time.