I noticed that too, I think it might be just chain wax, road dirt, grime etc building up in there. Thats what it looked like to me any way. I can't figure where that much grease would come from other wise.
i do a major cleaning three or four times a year to get
rid of stuff like that. it also lets you get close to most
of the parts you take for granted ... a good way to asess
the condition of the bike. like the race mechanic said, "how
can you tell if its broken if its not clean?"