Asp.net / PunBB 1.3 SSO (Share authentication cookie)

Hi all!
I am an Asp.Net person but would like to use PuBB 1.3 for my forums in one of my ASP.net sites. It is already using ASP.Net membership. I need to get PunBB to read the ASP.net authentication cookie so users don't have to sign in twice (When moving from the main site to the forums). The problem: although I have good experience with ASP.net and MS products, I have never written a  line of php code. What is the simplest way of achieving SSO in this situation. Both my app and PunBB will be running on the same second level domain but on different third level domains:
e.g. punbbFurums.mysite.com and myapp.mysite.com

I have considered using the .Net version of PunBB but it is still in its infancy and many features are missing.

Any pointers will be appreciated.

Regards,
Joe