This is why you do a lot of homework before you get to that situation.
First, know what the industry norm is for the position - you can always start there.
Second, know what the press is saying about the company - you get a lot of clues.
Third, hopefully you have found a contact within the company and you can find out a lot about them. Is their pay generally low/medium/high?
Fourth, you can lowball and then when you go into negotiations tell them that (a) that number was a misprint or (b) the truth, that you lowballed on purpose and now you want to know what they are really offering.
Fifth, lowball, start at a low salary, find out what everyone else is making, and then demand a raise - if you have balls, that is.
Sixth, you can call the person who asked you for the offer and sweet-talk them into telling you what numbers they are looking for.
Seventh, you can mention whatever you're currently making and ask for a bit more.
I'm a boss and I use this question to find out what kind of person I'm dealing with. The answer I EXPECT is whatever the industry norm is. If someone gives an unrealistically high figure, I dump them because it is obvious that they don't know what they're getting into. When someone says "I'll work for free" I dump them for the same reason. I don't mind someone asking me NICELY what the heck I'm looking for - as long as they have serious credentials and experience, etc. And I'm impressed when someone knows something about our company and is able to start with that.
Good luck.