>But I already know HTML pretty well. I wonder if I need to know XML as well.

Definitely. The world is moving that way (been for a while now).
Mike and Jay have given you examples.

It is important to know it as it is everyday more needed for exchanging data with the non-VFP world. And web services are important, wether you have a use for them at present or not. You should learn a little about it, if nothing else, to be knowledgeable when the client, or the boss, asks.