How long has this feature been back?
The only thing I missed from KDE3 was the option to put different wallpapers on each of my virtual desktops. It's a dumb thing, but, something I missed.
Well, lo and behold. While going through some news or forum feed, I found out that it was already in KDE4. I gave up looking a while ago.
System Settings --> Workspace Behaviour --> Virtual Desktops --> Then check the box for different widgets for each desktop. I guess I never saw the wallpaper as a widget.
Anyway, I thought I'd share.
Separate wallpapers has been in KDE 4.x since about 4.5 or so. If you ever figure out the value of "Activities" on the plasma desktop, I'd love to hear a use case or an example. I've never used it.