question about concurrency implementation

I'm curious about the implementation of Concurrent Haskell in GHC and Hugs. Does access to values possibly shared among threads cost the same in Concurrent Haskell as in regular Haskell? I'm guessing the answer is "yes", because Concurrent Haskell is provided by default in GHC. If the costs are the same, does that rely on there being no true concurrency in the current implementations? How would the cost change if true concurrency were provided? Wouldn't thunk evaluation involve mutual exclusion? Dean Herington