joint venture? What joint venture? This would be great for Cosi shareholders (I'm assuming you, for one) and lousy for Panera shareholders.
Panera has bought chains in the past in order to enter markets, as an alternative to the expense of entering and competing with them. (namely, Paradise...but even including Panera itself, or St. Louis Bread when the company was Au Bon Pain) But turning Cosi around would not be cheaper. Panera already competes with them effectively--no need for an alternative.