Each PLEX bought on the in-game market and donated to charity represents 30 days of game time that someone in the world originally paid for with cash but that won't be redeemed into game time. Because of this, CCP can donate the full cash value of that game time to a registered relief charity without taking a big loss. CCP absorbs all management and tax fees to ensure that 100% of the donated game time's value is passed on to registered charities (in this case, the Red Cross).
To donate to the relief effort in Japan, EVE players are encouraged to buy PLEX on the open market and contract them to the character "CCP PLEX For Good" before March 31st. As usual, EVE Online's normally permissive rules on scamming have been suspended in relation to the charity drive. Anyone attempting to scam players out of ISK or PLEX using this charity drive will be dealt with harshly. With notable rich players and corporations able to potentially donate thousands of dollars' worth of PLEX, this charity drive is sure to raise a substantial amount of money for disaster-stricken Japan.