I was told by someone that, just as .NET started inspired from Java, WPF was inspired by a similar technology, as "Microsoft never innovates".
However, I can't find anything remotely close to WPF.
What particular technology did or could have inspired Microsoft to write WPF?
CSS, XML, HTML...
But truthfully, WPF is a great innovation in the GUI dev arena.
Looks like it's part 2d on 3d acceleration and part Glade (XAML). I'm not saying that they ripped off earlier technologies. The Linux / UNIX side of things use a different architecture for graphics acceleration (due to it's history and needs). Let's just say that there are a lot of ideas which get reimplemented in graphics presentation until they either succeed on merit (or on lack of ability to compete against them). With Microsoft, often it's the latter which rules the day.
The 2d drawing accelerated by 3d has been done several times, mostly by toolkits meant for displaying menus and buttons in 3d accelerated games. The oldest one I know about is GLUI which you can read about here. GLUI 2.0 goes back before 1999, I imagine that 1.0 was much earlier.
Glade probably pre-exists WPF, but by no means is it the pioneer. Other pre-XML layout formats basically were XML-ified in the great XML cures everything days. Perhaps Glade was the first XML based, perhaps not. Glade dates back to 1998.
In any case, the differences in GUI architecture between Microsoft and Linux means that while a few key ideas might predate Microsoft's implementation on the Windows platform, Microsoft really didn't rip off the other projects, as their code wouldn't run on Microsoft's platform due to the architectural incompatibilities.
I realize WPF is more than just templating, but I'm sure it took some inspiration from existing UI markup languages. Many of these markup languages were a huge help in separating concerns and made it easy to apply consistent styles across a UI.
As far as separation of concerns, WPF uses a Model-View-ViewModel architecture which has been around in one form or another for a long time: