It used to be fairly common in 2e, but in 3e it has been pretty rare in the campaigns I've been in.
My Houserules tend to limit multiclassing (it requires 6 game months of training to gain a new class and you can only be a Bard, Wizard, Sorcerer or Psion if you were born with that ability), so I expect it won't happen too much when I start the next campaign up this fall.
The first exception is Fighter (or Warrior), which may be picked up after six game months of Adventuring experience.
The second is if the player worked at it, I would accept an apprenticeship to another PC for that time to gain the class.