The official site is a good start for those who can read Japanese.
I can start with the special ways to unlock certain songs...
I will give more details later...
Unlocks during Normal Mode/Challenge Mode:
(These songs can also be unlocked through the roulette system.)
Telephone Shopping, Symphonic Metal, Fairy Tale, and Rakugakids.
Unlocks during Expert Mode:
Marathon course: Anime Hero, Swedish, and Ninja Hero.
The Four Great Civilizations course: Kyougeki, Pyramid, and Tabla'n Bass.
Other unlocks:
Christmas Present. (Unlocked if the PS2 clock/calendar is on or beyond December 24, 2004.)
One question, will this FAQ be able to handle Japanese text, such as Shift-JIS encoding or something?
EDIT: I almost forgot, my e-mail is:
shungokusatsu_iidx@hotmail.com