WinPkFilter IM Driver seems to work with Vista
Yes, it does. Just need to fix the conversion of internal adapter name to user friendly one, but it can wait till Vista release.
I have signed the driver image, but Windows still complains about an unsigned driver. Do I have to package inf and sys files to a .cat file? Perhaps anybody has a short hint for me?
Have not tried myself yet, but as far as I understand you have two options, one is creating .cat file and another is embedding signature into the driver image. This papaer from MS covers some of the details:
http://www.microsoft.com/whdc/winlogo/drvsign/kmcs_walkthrough.mspx