A post on John's blog talks about a open-letter to Google from a blog that covers the porn industry: An Open Letter to Google RE: Online Payments. It asks the Googleplex to "embrace" the industry since PayPal hasn't.
I'll leave that discussion to others, at least for the present time. However, it's worth noting that all major search engines to our knowledge carry porn ads already. Google's even been criticized for taking porn ads but not gun ads (an issue with Yahoo, as well). And as Danny wrote yesterday, sex and porn still seem among the top terms searched for. Handling porn payment transactions is another thing entirely, of course.
If Google were thinking about getting into porn in some way, at least it's got a good domain for it. GooglePorn.com was registered by Google in 2002. It also owns Google-Yahoo-Porn.com, registered by Google in 2003. Perhaps it was thinking of a mega merger back then and preparing on the porn front.
It's more than likely that Google registered these and other domains as a defensive measure (check out YahooPorno.com to see what I mean, but not if you're a sensitive person) and to perhaps help with spam reduction efforts.