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).
The Rugged Motor Driver
: two H-bridges, more power than an L298, fully protected