You can airblock only when you jump backwards. You can block normal moves and fireballs, but you cannot block (S)DMs (i.e.Ryo's giant fireball of gayness, maiden masher, etc.) But, like FDPL said, they got rid of airblocking after '98. I personally think forward jump airblocking sucks *cough*mvc2*cough*.
[ August 06, 2001: Message edited by: ThE RuLeR oF EaRtH' LoRd YaMaZaKi ]