They usually don't reveal the cast until around a week before the show starts. Once each houseguest finds out they're a houseguest, they don't have much time before they're put into sequester, and then they're brought into the house. They're generally already in the house for three or four days before the season even premiers.
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http://i45.tinypic.com/fpchec.gif
http://img149.imageshack.us/img149/9413/cutepuppy16.png - Number of people that D'awwed: 60