the safe thing to do is to get the offer first, and negotiate if possible.

it is not tricky, it is just a trick they play on you.
even if they insist, it is safe NOT to give them a specific number.
because, 9 out of 10 they are trying to use this question to filter
out candidates that are at the same level of competency. IF you give
a number, there are a few possibilities:

1) they don't want to get a number at all, they just want to see how
you react to see whether you are more interested in money or the position
itself.  most of the time it is the case.

2) they do want a number (at least a range),
and
a) if your number/range falls
out of what they are expecting (say, a budget of at most $60K a year), you 
are kicked out of the pool of the candidates

  b) if your number is within their range and they don't mind your giving a
number,
it is OK. But if there is another equally competent candidate whose answer is
making them more comfortable, you are also out of the picture.


to summarize, most of the times, giving a number will reduce you chance of
getting a final offer.    the safe thing to do is to get the offer first, and
negotiate if possible.