The article in question points to two very revealing projects within Microsoft's R&D labs that have marked similarities to the Courier. The first is its prototype tablet PC interface called InkSeine. InkSeine works on a strikingly similar premise to the UI shown in the demo video, including a heavy reliance on pages in "notebooks," and the ability to scrapbook pieces of websites by screen-capping certain areas. InkSeine also bares resemblance to the Courier interface in its use of circular context menus which surround bits of content. While there are clearly major differences, it's easy to see a lineage from this interface to the more polished iteration from the Courier demo.
The second item of note is Microsoft Research's Codex Project (a project we covered back in October of 2008). This is perhaps more remarkable than InkSeine, as it is a nearly identical two-screen tablet like the one shown in the demo. While the prototype version of this setup is much less polished and finalized than the render seen in that clip, it's clear that there is a connection at Microsoft to this piece of demo hardware, and the mocked-up version in the Courier video.
Most interesting of all is that these two projects have been put into play in tandem, with InkSeine running on the Codex Project's dual-screen tablet. Even photos from Microsoft's Codex page show the two projects used to compile what looks like an artist's scrapbook for a project -- almost the exact example used in the Courier demo. Whether or not Courier ever comes to light, seeing these two projects from Microsoft's recent research past tells us two things: the first is that it's very possible some physical prototype of the Courier does actually exist, and the second is that Microsoft has been plugging away at these ideas for some time now. With rumors that Redmond is planning something post-CES (and supposedly post Apple tablet), it might only be a matter of time.
Read - Microsoft's Courier Tablet details
Read - Codex project
Read - InkSeine project