I don't know that there's a one-size-fits-all category of "monoculture." We know the extreme example, IE6, but does webkit deserve to be put in that bracket?
For one it's not solely influenced by one "culture." Adding Opera developers adds to the mix, doesn't it?
It also differs from the IE6 extreme because there doesn't seem to be a lot of evidence (so far) that we're getting stuck on one version of it. Is it going to be the case that we'll need to go dig up some ancient unmaintained version to keep compatibility with some mission critical junk that no one can fix? From what I see everyone's updating to newer versions of webkit Willy-nilly and not looking back much.
Maybe I'm not buying into the right paranoia but is monoculture an overstated problem with open source software in active development?