http://blogs.reuters.com/felix-salmon/2011/06/24/upgrading-s...
Basically, Skype's claim is that the point of this clause is to retain employees -- you don't get cash in your vested stock unless you still work for the company. But as Salmon points out, none of the employees actually realized this -- the clauses are bafflingly worded, and refer to external documents, to the extent that you'd need a lawyer to figure them out -- so it doesn't make any kind of sense as a retention tool. Add in the fact that this policy is pretty radically different from standard tech industry practice, where if you're vested you're vested, and it really seems like it's just a way to screw employees.