Bobby_M posted a thread about a week ago using a year old properly stored tube with a starter.
I've used nearly a year old tube (it was a couple months outdated and sat in my fridge for another 6 months or so) myself with no problems...The biggest thing besides applying the puke test with using outdated yeast is to Plan enough time to incrementally build up a huge starter.
Just like we can build up a starter by harvesting a scant few cells out of a bottle, outdated yeast can be used, because you will reproduce the healthy still alive cells.
My lhbs sells outdated tubes and smack packs dirt cheap....
Oh and there's this whole pre-historic yeast thing going on...
Food In The Fort » Old yeast. New Beer.
So anywhere from a year, to 45 million if stored properly.