Bobby M recently did a test on year old stored yeast here; http://www.homebrewtalk.com/f13/test...bility-126707/
And my LHBS cells outdated tubes and packs of yeast dirt cheap 2-3 dollars each
and I usually grab a couple tubes of belgian or other interesting yeast when I am there and shove it in my fridge. and I have never had a problem with one of those tubes.
I usually make a starter but I once pitched a year old tube of Belgian High Gravity yeast directly into a 2.5 gallon batch of a Belgian Dark Strong, and after about 4 days it took off beautifully.
You just need first to apply the "sniff test" if it smell bad, especially if it smells like week old gorilla poop in a diaper left on the side of the road in the heat of summer.
Then make a starter, and if it takes off you are fine.