I just received my Wyeast smack-pack, and it appears to be puffed up already. Is this an indicator that it has already been activated?
If it is going to be a few days yet until you plan to brew, put the yeast in the refrigerator, which will cool down your yeast and allow it to slow its activity.
Swollen packages are almost always the cause of a small amount of sugar or CO2 being left in solution at the time of packaging. Upon shipment, CO2 can be released from solution or the yeast can consume the sugar and create a small amount of CO2.
Cell autolysis, or cell death can also be a cause of swelling packaged. However, this is only in rare cases where the yeast is exposed to high temperature for an extended amount of time.