Open source solver for large mixed integer programming task?
Never done stuff on that scale, but as no-one else has jumped in yet have you seen these two papers that discuss non-commercial solutions?  Symphony and COIN-OR seem to be the dominant suggestions.

Linderoth, Jeffrey T., and Andrea Lodi. "MILP software." Wiley encyclopedia of operations research and management science (2010). PDF version

Linderoth, Jeffrey T., and Ted K. Ralphs. "Noncommercial software for mixed-integer linear programming." Integer programming: theory and practice 3 (2005): 253-303. Compares performance