The final of the five U.S. carriers to sell the Samsung Galaxy S III, U.S. Cellular, has started shipping out their preorders. Additionally they indicated that retail availability will begin tomorrow. Here's a quote with details,

Starting Wednesday, the Samsung Galaxy S III will be available on U.S. Cellular's website and in its stores by the end of this week. Initially, only the 16GB model will be available for $299.99 with a signed 2-year contract. Of course, there is a $100 mail-in rebate which knocks the price down to $199.99. The 32GB model is expected to be offered in the future.
Source: PhoneArena