As I said, Googlers were spam fighting over Christmas and it is interesting to see Matt Cutts chime in on the forums. In fact, I believe he is most active in the Google forums on holidays.
That being said, one adult site complained on Christmas day that his site no longer ranks. He posted his complaint in the Google Webmaster Help forums. Google's John Mueller responded quickly saying:
It looks like your site sometimes does a redirect for users, but not for Googlebot. This is considered cloaking, and against our Webmaster Guidelines.
But Dimita, the site owner, was not intentionally cloaking. But the site was indeed redirecting users and not GoogleBot. It seems like a hack.
Matt Cutts, Google's head of search spam, came in shortly after and dropped a hint that it might be related to PlugRush, an adult ad network and traffic program. Matt said:
Hmm. Were you using something like PlugRush? You might look into mobile redirects.
It took me some time to figure out what PlugRush was because if you Google it, the web site does not come up. Showing that PlugRush itself is penalized by Google.
So it seems like if you run an adult site and want Google traffic, it might make sense to look into what is going on with PlugRush, if you are running it.
Forum discussion at Google Webmaster Help.