I saw a few baby zombies spawn in my zombie farm. The wiki says it should not work this way, but this might be a new feature in 1.6.2. I have never seen this in a previous version.
You see the baby zombie with the other zombies in the screenshot
It is probably possible to reproduce this with another zombie spawner
I believe this is a new feature. It happens during natural spawning too. Baby Zombie Pigmen also spawn naturally.