The Japanese love their portable gaming and while it always used to be a one-sided contest between Sony and Japan, things have changed significantly.
According to 1Up citing the latest numbers from ASCII Media Works, the PSP was the best-selling game platform of 2010 in Japan. Apparently, it just squeaked past the DS and more importantly, PSP sales were way up over the lackluster 2009 sales: in 2010, the PSP sold 2,729,718 (600,000 more than 2009) and the DS came in at 2,719,544 (down by more than 1 million from 2009). The stats look like this:
You may also notice that the PS3 was only about 40,000 behind the Wii, and as per usual, the Xbox 360 is almost nonexistent. For the PSP, the release of the wildly popular Monster Hunter Portable 3rd very likely spurred the portable to its big 2010 showing and at this rate, it appears as if the PSP and PS3 will be #1 and #3 in Japan in 2011. Just a guess, given the current upswing/downswing trends with Sony and Nintendo.