I'm always at a loss as to why people won't wear the belts too.  I know of someone who fastens the buckle, and then sits on top of the belt!  He only fastens it because his car has a bleeper warning and this would annoy him otherwise.  

I've been to Greece a few times and it's not the law there that you have to wear them (as Ben mentioned, it is here in the UK and has been for a long while).  Taxi drivers can sometimes take it as an insult if you buckle up in their cars, they think you are questioning their prowess.

Take care.