The Japan post office has a notice, only in Japnese, see http://www.post.japanpost.jp/whats_new/2010/1112_02_c01.pdf
It says that from 11/17/2010, for an unspecified amount of time, there will be a restriction on the weight of packages that will be sent to the U.S. from Japan. The limit on weight is 16 ounces or 453 grams. The notice says it is for 'terrorism' related reasons. This restriction affects all type of postage, i.e., EMS, SAL, seamail, etc.
Order before this period if you want delivery.
If you want to see the Japan post office site and click the link yourself go here, http://www.post.japanpost.jp/index.html
scroll down a little to a "RSS" link and click the first link below that. It will take you to the URL I posted.