According to Nikkei Newspaper on May 19, 2011, government of Japan and Korea is likely to announce mutual recognition of AEO shortly. At this point of time, I find no official announcement on Japan Customs web site, but perhaps will be available soon.
As for AEO operators, 370 companies are registered in Japan such as Toyota or Nippon Express, while Korea has 141 companies. Ministry of Finance expect the trade between Japan and Korea increase in area of steel industry or semiconductor industry.
Japan have had AEO mutual recognition with New Zealand, US. Signed with EU and Canada. On progress with Singapore, China and Malaysia.
This news was officially annoucned on May 20 by MOF press release.