There are very few instances of old whisky going bad due to a reaction with the cork. It works much the same way that a 'corked' wine does, but the advantage the whisky has is that a majority of the time the alcohol kills anything nasty in the cork. As I say, I have heard of isolated instances but they are VERY few and far between. The general assumption that it will be fine hold true 99.9% of the time.
Jeff - I had some Tequila like that once.
