After much fruitless googling I came accross a modifed version 0.7 of libpcap that was ported by Nevil Brownlee to the cygwin environment. It allows you to use the unix libpcap API in your code and then translates the calls into equivalent WinPcap calls.
The port was conducted by CAIDA Metrics Working Group project to support the port of NeTraMet to MS Windows.
Source is in libpcap-0.7n.tar.gz at the NeTraMet downloads page.
The code is a bit dated (early 2002) and doesn't sync up with the current version of libpcap (v0.9.8). I don't really need it at the moment but it might be worth looking at later.