Running wp core update --version=nightly now updates to 4.9 alpha, since core branched 4.8 a few days ago and trunk is now 4.9 alpha.

If the WordPress Beta Tester plugin is used and is set to Bleeding edge nightlies it sticks to 4.8-RC2-40868. It appears to do this by appending a _beta_tester=true flag on the update request (source).

It would be great if WP-CLI supported a version flag such as wp core update --version=beta which stuck to the current beta/RC in the same way that the beta tester plugins appears to.

Might be worth pinging Westi or Dion for clarification.