Under "Advanced Search", there is a multi-tag search form field for searching by multiple tags; however, it uses implicit 'OR' functionality only. There is a request for added functionality withing the Advanced Tags plugin (which we use here).
I've looked at the advanced search but can't seem to find a way to search for a specific tag. What does work is to select a certain thread with a specific tag and then I get all threads with that specific tag. But this is limited to one tag. I think Z said its not possible to search for a specific tag but just wanted to make sure.