Surely it's just about ease of use? If people were forced to choose a four digit pin when they set up their voicemail, surely the polar ice caps would melt even quicker and all sorts of other terrrible things would happen.
Well, that's a novel reason for global warming. And there was me thinking that i'd heard all the reasons for it before!
Rich, yes… this is my understanding. Apparently many telcos don't require you to set a pin, and leave them as default 0000 (or similar) thus letting anyone easily access your voicemail. Hardly front page news.
I've also read that they would call into telcos and social engineer them into either resetting or giving enough information that the password could be derrived from said information. This is also called a pretexting. CNN reports that this may be another avenue that was used.