What would be best, forums.mysite.com or mysite.com/forums/ the latter I would think would share some pr right? am I looking at this wrong? any advantages either way?
I will answer the long awaited answer. Sub domain it is better it will cause the search engine to think they are two different sites and give you more back links from experience.
Sorry mate, I would say otherwise. I would only build a sub-domain site for branding purposes, but having a sub-domain will split the links between the main domain, and the sub-domain, as they appears as 2 different sites. You will gain PR and build link equity more efficiently if you use a sub-folder scenario.