Japan has issues. A rapidly aging population is set to bankrupt the country. The Japanese have maintained a high savings rate for years, but this phenomenon has ended. The post-war Japanese baby-boomers are retiring in droves. Retirees do not save, so Japan needs another way to finance its large budget deficits.
Either the Bank of Japan will have to let rates rise to attract foreign investors, or it will have to print lots of yen. Higher rates will lead to even larger financing shortfalls as the interest expense eats up current revenue, which is just like the death spiral we have seen in Greece and now Spain.
Before shorting the yen and Japanese government bonds, remember that we are still waiting for the euro to crash.