hi!
I'm using seperate activities for each workspace, because I like having a seperate set of plasmoids an different wallpapers for each workspace. however when I'm using the present windows effect i noticed that while the effect is on, I don't see the wallpaper of the current desktop, but the one of another workspace.
any idea's? or is this a known bug with kde (4.3)

This definitely sounds like a bug in the interaction between Plasma and KWin. Please file a report at bugs.kde.org aganist Plasma.

done:
https://bugs.kde.org/show_bug.cgi?id=212065