In a 1.2 dev image, when a user chooses System>Software update, they see a dialog explaining how to get an updatable (i.e. core trunk) image (issue ). This link should be the unique URL that was just set up - http://www.pharo-project.org/pharo-download/unstable-core.
Fix in PharoInbox: SLICE-Issue-2856-DevSoftwareUpdateShowsUnqiueCoreURL-SeanDeNigris.1