Assistant manager at my local Walmart called me and told me that she got an answer back from UPC/Sales support - release date is supposedly October 7th.
Take it with a grain of salt, but it's the closest thing we have to a release date at this point.
UPDATE:
The store manager just called me (about my issue on Saturday with the failed purchase...cant knock Walmart on their followup customer support) and said it's supposed to be in stock by October 7th, with an on the shelf date of October 10th (cant make this stuff up).
Just a heads up.
This corresponds with the release date for the new Roku devices.
https://www.walmart.com/ip/Roku-Express/54649024