Quite a long time ago support for NAT-PMP devices was contributed to Mono.Nat. As I didn't own a NAT-PMP capable device, I had to trust that the code was good at it worked. From all reports, it did under most circumstances. However it had a couple of issues which prevented it from being enabled by default. Patches to fix these issues have finally been contributed and have now been merged into Mono.Nat and released!
If you want to forward ports from your application via your upnp or nat-pmp capable routers, look no further, Mono.Nat is here! There are also packages available from the opensuse build service project I have.
If you have any issues, submit a bug report and I'll get on them!