I have never seen this, and I have used the Japanese Font Packs since Acrobat Reader 4 on many computers.
How does that manifest itself - the font pack "missing" but still visible in Add/Remove Programs?
How was the font pack installed originally
- explicitly - via the font pack download from http://www.adobe.com/support/downloads/detail.jsp?ftpID=3979 ?
- implicitly - from a Adobe Reader prompt when a Japanese PDF was opened?
When opening a pdf with Japanese characters, we get an error stating
the font pack is not installed.
The font pack installation must somehow have gotten corrupted during the Reader update. But how and why this happened I don't know, as I have never experienced it myself. I think there is a way to run Adobe Reader install or update so that it creates a log file, but I do not know off-hand how to do that. Maybe you can find that information in http://www.adobe.com/devnet/acrobat/pdfs/deploying_reader9.pdf