Network I/O is done using the usual suspect - which is WinPcap. There's no need for the WinPcap Developer's Pack! An ordinary WinPcap install - as it is i.e. part of Ethereal - is sufficient.
The code is still under development. But as I recently successfully opened HTML pages with the Contiki web browser I thought it might be the right time for an initial commit.
Please note that the VC++ 2005 Express Edition combined with the Platform SDK - which are both free downloads - are sufficient to build, run and debug Contiki on Win32.