Talk to your dealer about Technical Service Bulletin #050138016. This TSB is for "WATER LEAKING ON FRONT PASSENGER SIDE FLOOR OF VEHICLE, WATER BLOWING OUT OF VENTS (SEAL HEATER CORE COVER)." I recall reading elsewhere that the fix for this is to blow-down the line that drains from around the heater core as it may become clogged, and to replace the seal around the heater core. I am going to attempt to go to my dealership on saturday, but it may have to wait until next weekend while I am out of town and don't need the car. I will report back.
09 Cobalt LS 2.2L 5spd Silver Ice Metallic