When IDNs first started I remember most of the big registrars offered IDN Domains. I know Network Solutions did. (I knew about IDNs back then but wasn't into domaining)
The problem was the whole IDN concept was good & useful (for Japanese) but the main browser IE didn't support IDNs. They were selling a product most of the targeted users couldn't use yet. Most of the big companies in various countries held on to their registrations throughout the years. (Tons of Japanese companies have really premium Japanese IDNs)
It's now like 5 years later & IE7 is just getting IDN support. With the launch of IE7 now in public beta perhaps more registries will add IDN Domains to their inventory. In the case of Japan most popular Japanese registries offer Japanese IDNs. The few that I know that don't are web hosting companies that offer domain registrations.
(Hopefully this helps ya)