This thread is getting sillier by the post... so I'll contribute
Opeth's My Arms, Your Hearse and Still Life are both concept albums. For the stories, look on the FAQ. Ghost Reveries was originally intended to be a concept album, but Mike wrote Isolation Years and that screwed the concept. Again, all on the FAQ.
Concerning Ghost Reveries, it was concepted as being a Concept album but with the contribution of Isolation Years, the concept could be conceived as being contrived containing the content. Got it?
I think the problem is defining what a concept album is. SL and MAYH are both almost beyond concept, because their is plot development. Ghost Reveries I would call a concept, because it's a collection of works that share a common particular theme. Compared to an album like Deliverence, it much more conceptual.
Only joking mate... just a lighthearted stab. I think you're right though... I always think of a concept album being a story, though I know it's any album where all the songs contribute towards a common theme as well.