When mobile marketing first came out there wasn't enough sites that were mobile compatible and mobile browsers were shit. Even mainstream tried to go down the mobile road and found the same road blocks. But since mobile browsers were improved and since the launch of iphone/itouch the consumers are all jumping on the band wagon.
Consumers are using their mobiles for everything from games, email, surfering the web and buying via mobile. In mainstream I am always asked, 'Can we sell your site to mobile consumers / is your site mobile compatible'?
Remember most mainstream sites can only allow usa traffic. Adult sites are global and like harv said there are some countries that are more advanced in technology and have a shit ton of mobile traffic.
This is def one market that is growing and will be growing for a long time.
