AOL (and TT) users do appear to be those that suffer more frequently from MTU issues for obvious reasons. Eric is correct when he says that MTU should be set on the PC rather than the router as the router can be the point where incoming packets become fragmented if the MTU on the router is lower than on the local PC.
It can also be dependent on whether the router is blocking pings. If you make your router as unpingable, then this stops ICMP totally. Turning off ICMP breaks Path MTU Discovery.
Therefore AOL users not using a router*, or a router that is pingable are unlikely to notice the blackholing effect that can occur, since Path MTU Discovery will do its job correctly and adjust the end to end packet size accordingly.
*Some software firewalls on local machines could also block ICMP.
Despite what the likes of GRC says, theres many which say that turning off ICMP is bad and it "breaks the internet". So what if your router is pingable?