When it's that time in the interview to talk money I always asked for what I considered was fair given my experience and knowledge. This amount is negotiable depending on the job. If the job is good and I like the company, people etc, I'm willing to work for less than that amount.
But I also have a minimum amount under which I'm not going to descend no matter how good the job is (have bills to pay after all).
So I have amount X as the minimum I'm willing to work for and amount Y for what I consider fair given my knowledge. So I'm going for a salary that's greater than X, preferably Y or higher (if they give me more I'm not gonna' refuse after all :)).
But amount X is for me only to know and never disclosed it so far. Until the question was asked of me...
I interviewed for a small company and they said that they are impressed with my skills but can't offer me Y because they can't afford it so they asked what was the minimum I'm willing to work for. I know this wasn't said to trick me, I know for sure that they really couldn't afford to pay an employee amount Y.
I didn't tell them the amount and asked instead to see an offer from their side and then go from there...
So, should you tell a possible employer what's the minimum amount you are willing to work for?