If there's an easy way for me to allow my son to browse the net with little supervision, while minimizing the chances that he doesn't mistakenly happen upon 2g1c one day, I'd be thrilled.
Um yeah.
This domain name scheme isn't going to work like that. 2g1c and things of its ilk are pranks - things that your kid's idiot friends will show him. There is just about nothing more resourceful on this planet than a 13 year old kid who wants to find a way to gross out a friend for a prank. Even if this domain scheme could actually manage to put all of the porn on the internet behind a single 3-letter domain, your kid's friends will find a way to emotionally scar him. 2g1c is just a means to an end in that respect.
But to get back to the question of "protecting your kid from porn" (rather than "protecting your kid from the gross-out"), think about how ridiculous the whole idea sounds - given the Internet as it exists today, do you really think it's possible that somehow this scheme will manage to move ALL OF THE PORN ON THE INTERNET behind the .xxx domain? It's a ludicrous idea - each country controls their own domain space, and so they'd all have to buy in. And then, if your kid is even halfway intelligent, he's going to quickly figure out that there are these things out on the internet called "proxies" and that he can get into the porn ghetto pretty damn easily no matter what kind of domain name filtering you're having your ISP do on your behalf. And if you're relying on a local blacklist of the .xxx domain to keep the porn away from your kid, he'll figure that one out even faster.
As a nice side benefit, your kid will probably know quite a bit about how computers, the Internet and TCP/IP work before leaving High School. So there are a few merits to this approach.