GM has been dealing with this battery-caused corrosion for some time, they even have a repair kit & tech bulletin in place for this condition:
"Floor Pan Repair: Owners of some 2000-05 LeSabre, DeVille, Aurora, or Bonneville vehicles may comment on corrosion in the floor pan in the area where the battery is located. This may be caused by acid weeping from the battery, or from a dislodged battery vent tube. A new repair floor pan kit p/n 88948594 has been released to assist in the repair of this condition. Refer to Bulletin 05-08-61-002 for repair procedures and part number information. Clean the area of the floor pan where acid may have been present, using a solution of baking soda and water. Rinse and dry the area thoroughly."
I guess we'll have to start pulling the rear seat at oil changes to check on the battery, ugh!