This might me pretty old stuff for someone but I think it is anyway quite interesting phenomenon worth of mentioning of very pure metals, like tin or zinc. When I first read about whiskers, I out of interest grabbed some random D-connectors for closer inspection under the microscope at work, and there it was - very thin filaments of tin sticking out from the tin plating. Not one, but several ones. No question about it.
NASA has quite nice site about this, with large collection of pictures.
Problematic aspect about this is that whiskers make short circuits when the whisker grows long enough. This growth may take some time, like years. Although thin, they can still withstand enough current to cause problems (see the relays destroyed by whisker initiated arc flash!). NASA site says that even nuclear reactor was shut down due to whisker short circuit, kinda scary. I believe RoHS has make situation worse since lead was banned. Lead essentially prevents the whisker growth.
Regards,
Janne