Sony Computer Entertainment Inc. (SCEI) today announced that the cumulative sell-in units of PSP handheld entertainment system (PSP-1000 and 2000 series) reached a total of 10 million units in Japan as of July 15, 2008.

The Sony PSP was launched in Japan on December 12th 2004 in Japan. The slim and light PSP (PSP-2000) made its debut on September 20th 2007 and in the first half of 2008(according to Enterbrain), it has become the best selling system in the game console market in Japan.
On the games front, there are over 580 Sony PSP titles now available in Japan alone.
Via this Sony announcement.