PostgreSQL memory spike upgrade from 8.2 to 9.1
In a case like this where nothing significant has changed I think you are best off bringing this up on the PostgreSQL email lists.  Not only will you get help with your issue in terms of workarounds but if there is a planner bug somewhere the development team will be given notice so they can fix it.

This may be one of those cases where the level of help you can expect there is far better than you can expect here.