Does active dry yeast really expire? I was cleaning out my spice cabinet and deep in the back I found two packages (e.g., six envelopes) of active dry Red Star yeast that expired in March 2009, 15 months ago as of writing this question.

With the thought that dry yeast is basically freeze-dried and should have a decent shelf life, I proofed one envelope in warm water with some sugar to see if it really "expires". It foamed right up, so it seems like the answer is, at least for one year after the marked date, no.

Although in this case, does "expired" really mean "less effective" or "will taste funny"?
I've used yeast that was even older than yours and although the taste of the resulting bread was fine, and it foamed up properly when tested, I found I had to use about 50% more of it to get the same density of the bread.  In the end, I threw it out because it was too much trouble to experiment with it every time.