Weekly sales charts for Japan show the PSP as second-best gaming hardware in that territory. The top-selling platform, of course, is the Nintendo DSi. Here's are the full charts - provided by Media Create:

Hardware chart:

01 Nintendo DSi - 42,199
02 PlayStation Portable - 32,865
03 PlayStation 3 - 29,977
04 Nintendo Wii - 25,917
05 DS Lite - 6,550
06 Xbox 360 - 4,470
07 PlayStation 2 - 1,951

Software chart:

01 Pokemon Heart Gold/Soul Silver (DS)
02 Wii Fit Plus (Wii)
03 Tomodachi Collection: Friend Collection (DS)
04 Inazuma Eleven 2: Threat of the Invaders (DS)
05 Rune Factory 3 (DS)
06 Forza Motorsport 3 (360)
07 FIFA 10: World Class Soccer (PS3)
08 Wii Sports Resort (Wii)
09 Uncharted 2: Among Thieves (PS3)
10 Shin Sangoku Musou 5 Special (PSP)

Source.