The second beta of the next generation ArcGIS API for JavaScript is now available.
Version 4.0 beta 2 of the ArcGIS API for JavaScript has been released. Read about it in the blog at http://blogs.esri.com/esri/arcgis/2015/11/19/arcgis-api-for-javascript-4-0-beta-2-released/ or check out the details in the Whats New.
This includes support for vector tiles, a new popup design, support for local scenes, reading of web scenes and more.
Steve Cole - I'm not sure I understand, could you provide some more details or a screenshoot?
Using https://developers.arcgis.com/javascript/beta/sample-code/get-started-popup/live/index.html this is what I see:

Ok, my bad, I was reacting to the screenshots in the blog post where the popups are HUGE in comparison. You're right- in the actual samples, they are similarly sized to the previous API versions.
I do prefer having the colored title pane from previous APIs. Will this now require a CSS adjustment or will you be providing options within the API?
You can set the background-color property for .esri-popup-header class via css.