Well, your question isn't really an ASP centric question so to speak. At its most basic level all a forum is is a relational database once you have the database figured out and optimized, writing the front end is rudementary because all it has to do is make calls to the database and display data and allow for the insertion of data.
Of course you have the user management part of it but even that is rudimentary. Really, once you wrap your head around that concept, writing your own shouldnt be to difficult.
"The one language all programmers understand is profanity."