walkman - 7:39 pm on Feb 20, 2010 (gmt 0)
"Part of my site is a directory. It is not required to link back, however most people do."
Sorry but I don't buy that, most people don't unless they are not given a choice like: you don't have to really link but your link may be checked in 24 years. So you have been getting about 200 links a day for 8 years?
Looks like Google has flagged you and thinks that you're using the directory to get non-editorial links, it has nothing to do with 'no-follow'