My recollection is that the DOT rule requiring airlines to honor all fares - mistake or not - is still in effect. However, the DOT said as a matter of discretion it will not take steps against the airline so long as it makes the passengers "whole."

However, given that the rule is still there, the original poster could still sue for damages in small claims court. Damages could be the difference between the promised fare for the tickets and the current fare.