I have a point to discuss. If a sub-domain contain adult content then will it also affect ranking of main site which do not contain any adult content ? I came across a thread in official Google forum(http://www.google.com/support/forum/p/Webmasters/thread?tid=023e334920cef36a&hl=en), but not satisfied with this. So need opinion form Google experts.
i think it must take effect with the main domain, because our sub-domain is a subdivision from our main domain.
Do you think that respected direcotories Like Dmoz have adult category .. Why don't they got penalized even it is not on subdomain ?
for a new domain like we have now, google or search engine must think twice for allow us. but for dmoz or other site that have a big 'think' it's imposible, because he (google or SE) know he can got somethink valued from him (dmoz)
I think that if it's on a subdomain (like mentioned by the Google employee) the effect will be very low. I actually wouldn't break my head about it. If you are scared though, you could start 2 directories with a link to eachother (on the submit link page).
It's fine, it won't effect your main site. However, just not understand why don't you simply use a new domain?