I don't think that 'almost impossible' or 'virtually impossible' are high enough standards. The 'impossible' happens with quite regular frequency. Every week something 'impossible' ends up in the news: someone gets struck by lightning for the fifth time, someone wins the jackpot on a lottery for a second time, someone shoots a basket from half-court during a basketball game, etc. With enough Bitcoin clients running and generating addresses during the normal course of transactions it's just a matter of time before some monkey pounding on a keyboard comes up with "To be, or not to be". The only real protection (if you want to call it such) is to have many addresses in your wallet with all your Bitcoins spread among them. If/when an address is compromised you will potentially lose a little rather than everything.