I don't believe he has an exploratory committee yet. That is a formal move and he's made no formal moves.
Maybe you are refering to the "draft Fred Thompson" group, which isn't associated with Thompson.
I am talking about insiders to Fred Thompson, not the "Draft Fred '08" people. Exploratory in the sense of actually exploring the potential, not an official "Exploratory Committee" that is registered with the FEC. It is still too early for that.