I don't know about "the beginning of the end for unions," but I do think that it will be the end of that specific union. I bet it will make management a bit more bold when they're dealing with strikes, but this union made some pretty unique mistakes that I'd bet other unions will learn from. They definately bluffed and raised when the should have folded. For whatever reason, they didn't predict that NW would/could get replacement mechanics and cleaners. The union has also been so aggressive regarding its relations with other NW unions (aggressively getting new jobs for its workers at the expense of the other unions, recruiting members away from other unions) that it alienated itself from them and destroyed any possibility of a sympathy strike from other NW unions. Then, they overestimated the "union brotherhood" by thinking that people in Detroit (a heavy union/UAW area) wouldn't cross picket lines and would change or cancel their travel plans. Maybe really pro union Detroiters are booking on different airlines as protest, but it will be months before the flights not booked today actually take off half empty.
In short, nobody gives a flip about the union itself, and NW has offered pretty generous terms for letting the individual members return to work. I think both sides -- unions and management -- will be learning quite a bit from this one.
