There was a very similar question on Metafilter just yesterday[1], and this answer summed it up:
"Whenever I hear a candidate dither about specifying pay, I know they are either extremely flexible about pay, don't know the market very well, or don't value themselves very well. I know they will be willing to accept a lower amount than I would otherwise be able to offer.
In other words, if you don't specify a number, I'll specify one for you - and it'll be at the low end of what I can offer, not the high end. The highest paid candidates are the ones that know their value and refuse to work for less than their value. Most of those highest paid candidates are the ones that don't even bother giving a range of pay and simply say, 'I will work for $x'."
And, if it turns out your number doesn't result in an interview, then you've both saved a lot of time and a potentially huge dent to your salary curve.
So, as the above says, know your market and know your worth.