B&N is now the exception, because they are using "password" DRM on their ePubs. This will potentially be available to everyone (i.e. all reading devices and all ebook retailers) next year, but currently B&N ePub is B&N only.
You can buy from anyone else selling ePub and it will work on your Sony, but that was true before. The big advantage of Sony going ePub is that everyone (including Nook owners by the way) in the US can buy ebooks from Sony. I bought six from them this weekend. They have a different selection of ebooks than most other sites, so this helps the ePub infrastructure, even if they are not the cheapest. If more people buy from the Store then prices may even come down.
So far, nothing I have described is an advantage to Sony Reader owners, but in the long term the LRX format could not survive. No publisher is going to stand for paying extra for a minor format, so Sony would have to pay to convert from ePub to LRX. Since they already support ePub on their Readers, this change was necessary to survive (and I'm sure they got a good deal from Adobe for giving up LRX).
The geographic restrictions on ebooks are a real problem, made more apparent by the standardization more or less world wide on ePub. All that can be done at present by readers is to work out ways around them. Based on recent posts it seems reasonably easy for non-US customers to buy from Sony, but I have not been following the details.