I can only imagine the kind of domain name land-grab this will invoke. Squatters will be prepping their naughty keywords database in preparation, I suspect.
I think the cheapest way to get a desirable .xxx domain would be to register a nonsensical domain like "dumptrucking.xxx" and then make it a new term for some sex act. What is "dump trucking"? I don't know, and neither does anybody else -- yet. It's brand-building writ sleazy.
I think we can assume premium domain names will be sold in a land rush auction, not registered on a first-come-first-serve basis. Similar to e.g. .ASIA or .AM.
There may even be different registration fees like .TV has.