Ok after some more digging about in the code I have worked out that the app_code source doesn't actually compile in to a class library even when put in its own project and only compiled on its own.
It's like something physically prevents this code running under .NET 3.5 and any attempt to do so will either result in an explicit conversion of the assembly to a 2.0 compilation or the code simply left out of compilation altogether.
I've never seen anything quite like this before ... YAF is confusing .NET !!!!
I have noticed that the entire namespace "YAF.Editor" exists in a single class which may somehow be an issue (although I doubt it) so i'm going to separate out the classes and try and compile smaller parts of the namespace.
I should note that in my particular case I am trying to integrate the YAF code in to an existing solution where the application is becoming part of my CMS which may complicate things a little especially considering I am using some silverlight in my solution.
I used this ...
http://wiki.yetanotherforum.net/(S(0ewncg45w15z3ai4gc2irqbf))/1.9.3%20Integration%20with%20existing%20site.ashx
... to guide me in to the integration and have only integrated the binaries not the full source.
maybe if i take the full source and pull that in to my solution I can add a little light on the subject but the rest of the code appears fine at the moment, its only this YAF.Editor namespace that is kicking up any issues.
Having said that, without this code firing up I don't have any way of executing the rest of the code.
so ...
Why would a class refuse to compile ?