Not really law, but convention. .xxx is a Sponsored TLD, which means there is a consortium of sorts who will approve or deny applications for the domain name. There are rules and regulations for the registrants of the domain, and I suspect that part of those rules would be a registration fee of some sort -- a token one at a company level, but painful for an individual, say $2500 or so -- in addition to the normal domain registration fee of $10 or whatever it is. I also imagine that while there most likely will be an ibm.xxx, it will be a redirect to ibm.com.
One of the reasons that this has taken so long to be placed into action is how inefficient a system this will become. No one is required to register a particular tld. There are more rules keeping registrants out of tlds or forcing them to give up their current tld than there are forcing registrants to adopt a particular tld. Tlds were never meant to be implemented as a way to filter content.
Also, currently successful sexually themed sites are very unlikely to forego brand recognition. Why should mostviewedsite.com switch to mostviewedsite.xxx when the .com is the recognized brand name? Yes, they'd probably buy their .xxx counterpart, only to have it redirect to .com. In a very real sense, this new tld will likely double the number of adult-content sites, but not necessarily double the content. However, groups opposed to this sort of content will now be able to cry louder than ever that the internet is "full of filth."
It seems to me that the only winners in this are the registrars and the certifying authority. I doubt little else will change.