It's quite easy. I own http://sqr.cat/ for instance. You just register it through http://gandi.net/
The only requirement for .cat is that you need to display a page in Catalan for a minimum of 6 months.
The only requirement for .cat is that you need to display a page in Catalan for a minimum of 6 months.
Furthermore, they clearly say that violation of the only usage restriction (contents in Catalan) can result in your domain being suspended.
edit: Just a little correction to your text in Catalan: L'idioma català és una llengua bonica. ;)
Actually if you try to add a *.cat domain through gandi.net, they included this clause as a tip-text encoded in the little warning image: "Warning: the registration contract for domain names in this extension require that you publish at least one page in Catalan within 6 months."