I have tried the example in the documentation with Drupal 8.0 in beta12 as well as with the .dev-Package. I have enabled the modules (HAL,HTTP Basic Authentication, REST UI, RESTful Web Services and Serialization) and send the request via CURL and other HTTP REST clients to Drupal.
It is always returning HTML!.
The headers I am sending are OK, I checked that as well ("Accept: application/hal+json"). And I know the URL to access nodes has changed from /entity/node/... to /node.