And what was the most fundamental technology enabling “mash-ups” of desktop applications?

The clipboard. And a set of common clipboard data formats.

Before the clipboard, individual applications (such as Lotus 1-2-3 with its Copy and Move operations) enabled intra-application data transfer – in a world largely designed around a single running application. But the advent of the multi-application user environment, combined with the simplicity of the Select/Cut/Copy/Paste/Clear model, suddenly empowered the user in ways they hadn’t previously experienced.

More on this later, but for now it seems like the only official Microsoft resource on Live Clipboard is this mailing list. I'm sure there will be more discussion on MSDN later.