Did you see this post?
http://forum.m0n0.ch/index.php/topic,4547.0.htmlM0n0wall seeing your NIC is good, means the Hyper-V's virtualized NIC is recognized by M0n0. I'm assuming you're using the "Legacy NIC driver" in your guest setup? Interesting that you can ping no problem, don't know why you wouldn't be able to access the GUI. Can you run a Wireshark and verify the TCP handshake is proceeding as normal?
Joel.