It all depends on location. Until a few days ago, no stores here in the midwest had either new reader in stock except SonyStyle stores, which suck because they charge a 15% restocking fee in you decide to return it. And you only get 14 days to take it back.
Most Best Buys have the 300's, or are getting them in on the next shipment, but it'll be harder to find the 600 (they are backordered on their website and only available online, it says). You can order the 300 online and it'll ship in about 1 day if your local store is out of stock.
I talked to the people at Best Buy and they have a favorable return policy if you are still skeptical. They said it's 30 days and there is no restocking fee.
Like boca stated in an earlier post, the one problem with getting these in retail stores is that the people working there know nothing about them. You can go up to the dude working at Walmart and ask him about a Sony Reader and 9 times out of 10 you're going to get a blank stare and a, "What?"
I've had my 300 for about a week and I really like it. I have a near-sized photo, a review, and a couple of videos of it posted over here:
http://www.the-ebook-reader.com/sony-prs-300.html