This question gets brought up every so often. I've found two separate threads (several years apart) on the IxDA list:
I thought it was discussed on UXExchange as well, but I couldn't find it.
In my opinion (and it seems to be the general consensus), the icon is ubiquitous with saving. Changing it would cause more problems than it would solve. Think of it this way - can YOU think of anything to replace it with that would be more universally understood? There really isn't anything.
The same thing holds true for the "phone" icons used on cell phones and even Skype, or (eventually) the envelope icon for email. When was the last time you saw a phone that actually looked like the old, standard handset that is almost always used as the phone icon? I doubt most kids would even know what that icon was if it wasn't the button to talk on their cell phones. Yet, it is still widely known and probably will not be going away.