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Negative frequencies do not exist in reality.
Thus, they have no physical meaning.
However, they can be used - and are used rather often - for theoretical explanations or for calculation purposes, for example if you transform a lowpass into a bandpass response.
Added after 19 minutes:
By the way, on 26th of June 2008 a discussion on the significance of negative frequencies has started already in this forum.
They arise naturally in Fourier analysis because we are trying to express functions (sometimes pure real functions) as sums of complex exponential. There is no way to make a real function out of complex exponential unless we add two complex conjugates. exp(j*w*t) and exp(j*<-w>*t) are complex conjugates.
For examlple, try to make cos(w0*t) using functions exp(j*w*t).
It can be made by two components at w=w0 (positive frequency) and w=-w0 (negative frequency)
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