According to Variety magazine, we will get to vote from 20 contestants and the top three male and female vote-getters will be locked for the Big Brother house.  Producers will choose six more for a total of 12 guests this year.
 

The reason viewers will only get to choose half the contestants is so that the producers can make sure there is a balance in types of players in the house according to show co-producer Allison Grodner who was interviewed for the article. The article states that all 20 potential houseguests will gather outside the Big Brother house on July 6th on live TV for the announcement of the final 12.

Even though that is how it is being reported, my guess is the contestants will tape the bit so that the winners can enter the house and Big Brother can get a couple days of tape before the first official show.  This is how it has been done for seasons 2-6.

So tune in to CBS on June 21st at 8pm to see which 20 former house guests we will get to choose from from all previous 6 seasons of Big Brother.  Don't be surprised if your personal favorite is not among the 20, there have been 74 houseguests over the last six years.

My Big Brother 7 Page - WILL contain information that has not been shown on the TV show yet - so don't go there unless you want to know things in advance.