Only if they choose to redistribute it. If they just run it as a service where the code is never sold to it’s customers – either as source or compiled – then they are under no obligation to release the source. I doubt it serves their interests to release the theme and in my opinion that’s fair enough and in the spirit of the GPL.
Besides this, it is arguable whether they would be bound by the GPL or not. As a theme strictly speaking isn’t a modification of bbPress (it’s just separate code that is compatible with the theme architecture) and the idea of “linking” as stated in the GPL is pretty vague when it comes to PHP applications. It was originally devised to cover more traditional application models where linking to libraries is more clearly defined.