The problem here is that it's difficult to delay this question forever. So rather than delaying, you can address it in a way that makes it clear information needs to go both ways.
I read an article from a real estate agent about negotiating, and he had a pretty clever answer to a common question. He said people would often ask "would your clients take 10% below asking?". His answer "make an offer!" That certainly is one way to find out.
It's a good answer. The agent explained that what people are trying to do here is get information for free, without committing to anything themselves, yet the answer accomplishes this not by getting preachy or defensive, but by moving the negotiations toward what the agent wants to see most - an offer.
That should be your goal when an employer asks you what salary you'd accept or what your current salary is (a similar question in disguise). They're essentially asking for information without giving you any in return - they now know what you make, but you have no idea what they typically pay. You could try to stonewall for a while, but I think it's better to have an answer that moves toward an offer. If they ask what your salary is, you could say something like "I work for a small startup right now, so my guess is that a senior software engineer's salary at a big company would be higher - what is typical in your organization?" That way, you haven't said "no, I won't tell you", nor have you said "you have to tell me if I'll tell you" so bluntly. What you've done is posed it in a way that moves them toward putting a number onto an offer, and they will have to acknowledge, through their response, that they were actually fishing for free info to give themselves the upper hand here.
Now keep in mind, employers are clever, and odds are they do this a lot more than you do (not always, consultants in particular do negotiations more often than their clients). But keep in mind the principle behind the response "make me an offer." It's a way to remind the person you're negotiating with that they have information you'd like just as much as you have information they'd like, and that to get it, they're going to need to take a step forward themselves.
One more thing, if you're desperate for a job, it's much harder to do this. Ideally, you'd be in a position where you'd really like a job (if you're indifferent, why are you doing this), but coming form a position of relative strength. Also, keep in mind, if asking a question like this "poisons" the relationship with your employer, it's probably not going to be a good one anyway.