http://www.domaintools.com/internet-statistics/registrar-stats-2007.html

Worth considering.. all other things being equal, chances are sites (linking to each other for example) have a reasonable chance of being with the same registrar. At least they don't have to be unique all of the time.

Yep and I mostly stick with the major ones for this reason. Nice chart though and wow is gd a monster or what.