We are constantly told (most recently by Ted Kulongoski) that there is "no way" to deport 12 million illegal aliens from this country. (no word yet on whether or not we could arrest 12 million burglars, ID thieves, etc)
But the point is: WE DON'T ARREST THEM ALL ON THE SAME DAY. Just like we wouldn't need to deport everyone on the same day.
Please read the NumbersUSA page on Attrition through enforcement.