Mark Burnett has confirmed that the Amazon is indeed the location for S6. You can listen to a radio segment HERE.
He also says that S9 would be ideal for an all-star edition, where he would (probably) have 2 people from each of the previous 8 Survivors, and no celebrities.
Speculation is also going on that the location for S7 will be Mexico.
Filming is currently under way for S6.