Various ISPs have been trying lots of different ICMP filters. You can see some of the impact on the Internet average graphs from XAffire.
http://www.xaffire.com/press/ea/EA20030902_images?rf=EM005 Xaffire/Matrix Systems apparently used ping packets that were the same size as those being filtered by some ISPs. According to Xaffire service providers implementing filters included Cable & Wireless and Level 3.
