Can't say anything about Nexus10 support, but the webbrowser, it does support pinch-to-zoom. (I think it's also shown in the demo video.)

On a Desktop, it's orthogonal to virtual desktops. You can use Activities in the same way as virtual desktops, but while VDs are only suitable to group windows, Activies can group pretty much everything. This is of course not a coincidence. :)

Yes, the basic idea is that you group everything into Activities, which you can switch between. In such an Activity you can have running apps, app launchers, documents, images, links, bookmarks, widgets, contacts, messages. These "items" are all tied together by a semantic layer of metadata which represents your grouping as activities. You can easily add (or remove) those. Adding a bookmark to your current activity is for example as easy as tipping the connect button in the top panel, and then choosing "connect to current activity". It will then automatically be shown on your activity screen and "semantically associated" with the other bits of informations you have there.

I'm using an exopc as development machine, for end-users, ARM-based devices, such as the NVidia Tegra base ones, or the Archos G9 are interesting options. Basically, as long as we can get kernel (and driver) sources from the device vendors, and the device has an open boot loader, we'll be able to get Plasma Active onto it. Here are two links showing interesting devices that run Plasma Active: http://dot.kde.org/2011/11/30/plasma-active-archos-g9-tablet (Plasma Active on Archos G9) http://dot.kde.org/2011/10/24/plasma-active-arm (Plasma Active on NVidia Tegra 2)

In the MeeGo and Mer images, pinch-to-zoom works for the image viewer, the webbrowser, and other Plasma apps that implement this gesture. It's just not shown in the video.

As far as I recall, there's Summer of Code project dedicated to getting Quanta4 going. Basically, most of the things that quanta3 did can now be implemented as some sort of chrome on the kdevplatform (the framework that also backs the recently released new version of KDevelop). http://milianw.de/blog/gsoc-revive-quanta-brand-for-kde-4 has more details about that. Milian is also the guy who has implemented pretty awesome support for PHP in kdevelop, by the way.

That's a misconception. Plasmoids can both be installed through your system's package manager (just like basic plasmoids that make up the desktop interface are), and starting with 4.2 also through webservices (Those of course only works for non-compiled language plasmoids such as Javascript, Python and Ruby). A security mechanism for installing them through webservices is in place. It'll probably also be possible to share plasmoids among users, to just drag+drop them from a webpage onto your desktop, panel or sidebars. The system is quite flexible.