You'll be pleased to know that I had exactly the same question when getting told about this too.

After thinking it through, it makes very much sense. It looks absolutely crazy, but is actually a good idea for Nokia to be involved in meego.com this way. This is of course speculation, but it's how I can make sense of this.

Consider this:
* Harmattan is the basis of next device, not meego.com Core - see this post - let's leave this device out of the equation to understand this announcement, there's other people dealing with this one.
* There will only be one MeeGo device in 2011 (according to CEO/CTO)
* There was supposed to be a significant benefit cost wise by going for MeeGo.com as to no longer have to maintain and develop your own Linux distribution inside Nokia. This would have been an expected cut in R&D costs. Going back to maintaining your own distro would be stupid. So using MeeGo.com or something else would be only possibility.
* Since the 770 Nokia has been working to create a Linux platform, tried many many different UIs and toolkits, but now we're at a state where we have a lean and mean Qt+Linux platform that you can practically make any kind of UI on easily (QML, off-the-shelf Linux components) for any kind of device. As an example, we remade the MeeGo Dialer UI in 2.5 hours in QML, previously made over several months in MTF.
* MeeGo is going to be used for future disruptions supposedly. To make future disruptions, you need a solid platform (read: you need to contribute to it to make it such) that you don't have to reinvent the wheel and write thousands of components to even get a UI up on each and every time you make a product. This is obviously MeeGo.com. It would work with same APIs on all sorts of devices ranging from TVs, to netbooks, tablets, mobile devices and so on. Take a off-the-shelf device capable of running MeeGo.com, start prototyping and inventing. Innovate fast.
* Having a basic handset experience that just works properly enables other vendors to see the benefit of MeeGo.com and perhaps make their own devices.
* Having a cheap, small mobile computer (please note that I say mobile computer, not 'smartphone') that runs MeeGo.com is a needed thing for prototyping/innovation/general development as well - for both research, companies and individuals. Considering an Aava runs at 2000 EUR something, it's an obvious benefit
* Use ability to do fast hardware adaptation with MeeGo.com Core to decrease time-to-market for innovative new devices (take chipset vendor HW adaptation + add your customizations + MeeGo.com Core + your stuff = quick product).

In the end: DE is good way to keep MeeGo.com involvement going from Nokia side as well as provide ability for Nokia and even other companies to create future disruptions using MeeGo.com in long term.

I personally don't believe the future of computing is a walled garden. You would need a open ecosystem, open standards and open platforms to make something like this "A day made of glass" video work in practice.

So - consider this a great chance to get involved with MeeGo.com and shape the future of your devices and environment the way you want them to be like.