I think that really depends on what your site is. If you are on the affiliate side and setting up sites with the intent of driving traffic, getting ready for mobile can be as simple as having a mobile theme setup to detect mobile traffic and deliver them a layout appropriate to their device. In many ways, this can be accomplished with CSS and having a style sheet optimized for mobile devices. If you've got a wordpress blog, there are mobile themes available that do the detection pretty well and are set to go. Customizing them is a plus though, so they match the rest of your site.
You also need to look at what content you're delivering. Does it work on a mobile device? Are you pointing the surfer to mobile content that will also work on their device or do your sponsors support mobile well enough to work on mobile devices?
If you are running a program, you may have to rework some content to work on mobile devices. At that level, you really need to be thinking about how to sell to mobile surfers.
The only 3D things I'm aware of that are really out there have specific hardware requirements for the users. If you don't meet them, you don't get to play... I think until those situations are dealt with, so these types of limits are eliminated, that market can't spread too far.
I don't know that mobile is truly the future of adult, but I know there are plenty of people who do think that. I think for sure it will continue to be a larger and larger component of the adult market, so everybody needs to work on how to address it. It's a good thing we have nothing else to do with our time other than figure out new markets!
