>From "Scientists confront Creationism" Ed. Larie Godfrey
In order to test the hypothesis that cosmic rays might affect the decay rates, Maxwell (1928) took a polonium source 1,150 feet below the earth and found no change within the accuracy of the experiment.
Pressure, chemical state and temperature can change the decay rates but under these very extreme conditions, the maximum variation is a few percent.
Perhaps Vernon can help us out here and show us some data/research that indicates that decay rates might have varied in the past ?