For what it's worth:
I was in the Sprint store in Canton, OH on Friday night to get my 300 repaired, and was talking to the tech about the 600. He informed me that it is likely to hit the stores this week. I asked if there was going to be any special upgrade offers for 300 users, he told me that he heard at first, it will not be that great, but wait until around the 15th. That's when the offers will get really good. Who knows.