This is why you need to keep whisky upstraight. The whisky is to strong, and would dissolve the cork.
Unlike wine, the evapourated alcohol is enough to keep the cork moist, so it wont stop sealing the bottle properly.
I have seen discussions going on if whisky should have plastic corks, which is less "natural", but are in fact probally better for whisky then corks are...
[This message has been edited by Jeroen Kloppenburg (edited 24 January 2003).]