A simple way to do it is with a small microphone and amplifier circuit, like the one on our
Gadget Shield or a stand-alone module like
this one.
The question is are you just looking for a sound/no-sound decision or are you actually trying to measure the frequency of the whistle and only want to respond to a certain whistle frequency? Selecting a single frequency is a little bit more work, though there have been some Arduino FFT projects recently that can help with this (searching for "Arduino FFT" will give lots of results).
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The
Rugged Motor Driver: two H-bridges, more power than an L298, fully protected