It really is an amazing phenomena. Effectively atoms of tin are moving from the tin out to the end of the whisker. We spent a week talking about whiskering(sp?) in my materials science class at USC because it was such a big deal in the EE world.

Prior to that class my world view was that only electrons could travel through metal, only to find that metal can travel through metal too!

Locally at NASA Ames they had an experiment where they had a bunch of different assemblies being exposed to various conditions (high electric fields, non-ionizing radiation, etc) and one of the things they were measuring was the production of whiskers and other changes in material properties (strength, toughness, Etc.). Always amazing what we know and what we don't know about what we know.