If folks recall, there was a big debate about tiered/differential pricing in the .biz/info/org contracts, see:
http://www.circleid.com/posts/icann_...fo_org_domain/
Eventually those contracts were amended to prevent that.
However, if folks read the .XXX proposed contract at:
http://www.icann.org/tlds/agreements...mt-05jan07.pdf
Appendix S, Part 2, under "delegated authority" (page 66 of the PDF), appears to give the Registry Operator total control to make policy regarding pricing. Thus, it would appear they are in a position to re-price domains that later become successful under .xxx.
Since the registry is already planning to reserve premium generic domain names (e.g. toys.xxx) for special treatment in regards to pricing (e.g. sold through auction or other means to extract value), and given that the operators seem to rationally want to maximize their profits, it would seem to me that anyone planning to invest in .xxx domains should pay careful attention to their registration agreement, to be sure that they are able to renew their domain names at a reasonable price.
If not, then I can see a scenario where a domain that was once offered at $75 per year that becomes a popular website can suddenly jump to $100,000 or $1 million/yr for renewal. Or, maybe you'll have to bid at an auction to renew your own domain name, and your competitors can try to outbid you.....the rules seem to be open for ICM to develop in this regard.
If anyone sees anything in the contract that prevents this, and protects registrants, please post it. Given ICM's track record, I'm sure folks would want to see such registrant protection in place BEFORE any contract is accepted by ICANN.