I know HTML, what else do I need to know?
For a designer: Flash, DreamWeaver, FireWorks (or Xara X), PhotoShop, Director (including scripting would be nice). Some mark-up ability would be useful, though an understanding of what you should/can and shouldn't/cannot do with a web page is essential. On the other hand, I'd expect you to push the developers' skills; make them think "Well, no, that can't be done" and have them come back an hour later with a solution. A skill in illustration is good.