why filtering ICMP can break PMTU-D

I have had a few requests regarding just how ICMP messages not reaching where they should (for whatever reason, including using private address space that is filtered somewhere) can break path MTU discovery. Anyone interested who doesn't understand it should take a look at the brief description I have available at: http://www.worldgate.com/~marcs/mtu/ which tries to be as simple as possible and explain it in a way that almost anyone can understand. Was originally made for referencing in complaints to silly providers that break connectivity by broken filtering and can't understand the problems.