Yes, they can use any symbol. But I don't think they can match against a constructor (that is, write a pattern like (x y) where x can match any constructor).

Also, I don't think they can have dynamic patterns. That is, you can't take part of a pattern, pass it into another pattern and use the resulting combination of patterns to match against some value.

These two things are the interesting generalization of pattern matching that I was talking about.