Updating to Drupal 7.x-dev until next stable release I am currently using Drupal core 7.7 and Commerce 7.x-1.0-rc3 which requires changes that are only currently present in Drupal core 7.x-dev. 

If I upgrade to 7.x-dev now in order to get the updates that the latest Commerce version needs, am I committing my project to following dev branch, and is there a danger of not being able to come back to stable cleanly, when the next stable release comes about? 

It's not a live site, so temporary instability a dev branch may introduce is not necessarily an issue if I'll end up back on stable soon.

EDIT: Additionally, instruction for how to upgrade core from stable to dev using Drush would be super: Drush: Update core from stable to dev  :)
No you are not: the Drupal core development workflow makes minor versions just tags of the main 7.x development branch. Periodically, the branch maintainer decides enough things have been committed to the branch to make a release worthwhile, and a new minor version release is packaged.

So let's say you download a snapshot of 7.x-dev dated 2011-08-15. At the end of the month, Drupal 7.8 is released. Drupal 7.8 would contain all of the changes you had received in the 2011-08-15 snapshot, and anything else committed between that point and the end of the month. It's essentially the same as if you downloaded a new development snapshot on 2011-08-30.

So, if you need to use the dev branch, feel free to do so with relative impunity.