ugly.js

Kumo.AppsNewController = Ember.ObjectController.extend(Ember.PromiseProxyMixin, {
	projects: null,
	selectedProject: null,

	repositories: null,

	repositoriesGetter: function () {
		var self = this;
		return ORM.factory('repository').where('projectId').isEqual(this.get('selectedProject.id')).findAll().then(function (data) {
			self.set('repositories', data);
		});
	}.observes('selectedProject')
});

Why not do something like this??

Kumo.AppsNewController = Em.ObjectController.extend({
    _reposLoadSuccess: function (data) {
        this.set('repositories', data);
    },
    _reposLoadFail: function (error) {
        // do something now that this has faild
    },

    actions: {
        projectSelectionChanged: function (projectId) {
            // update your model
            this.set('model.selectedProject', projectId);

            // get new repos
            this.send('getRepositories', projectId);
        },
        getRepositories: function (id) {
            ORM.factory('repository').where('projectId').isEqual(id).findAll().then(
                this._reposLoadSuccess,
                this._reposLoadFail
            );
        }
    }
});

Kumo.AppsNewView = Em.View.extend({
    tagName: 'form',
    templateName: 'apps/new/view'

    projectsSelect: Em.View.extend({
        tagName: 'select',
        changed: function (e) {
            this.controller.send('projectSelectionChanged', this.get('value'));
        }
    })
});

and in your AppsNewView template

<label for="select">Project</label>
{{view view.projectsSelect}}