On KDE-3.5, the kicker panel can be made transparent. It's fine, except that when I move between desktops _with the keyboard_, and only with the keyboard, sometimes the transparency is lost. This makes the desktop look ugly. I'm using the keycodes Mod4 + arrows to move around, if this makes a difference. I hope this would be fixed in the future.
Then you need to tell the folks at bugs.kde.org as they're the ones who can fix it.