did you read the wiki?
you run into the upgrade bug.
it's in kdelibs

No, I should not need to read wikis every time I upgrade. Also, if pacman indeed has a bug (as I now read on the wiki), then the kde upgrade should have been hold off until the pacman bug was fixed. Right now, you just create inconvenience to your unsuspected users.

another way would have been for the kde meta-package to explicity do the erase of kde 3.3 and then do the upgrade. But under no circumstances the packager should expect all its users to read wikis before they do their daily pacman -Suy, which is now a routine.

So, how do I fix the problem now?

do you read frontpage news?
ok sorry forgot yesterday to update it to extra, but the news is there since march the 6th, i sent now a mail to ML more i cannot do.

oh forgot to answer on your problem,
do a simple pacman -S kde and all is fine again

Not often, no. I don't read Apple's news pages either, nor Microsoft's. Yet, their upgrades do work.

Tobias, I believe this is a honest mistake. It's just that it will create grief to a number of users because nobody should base an upgrade's success to the *chance* that someone might read a "gotcha" note. Most "normal" users don't read, they simply upgrade every few days and that's that. They don't have to be close to the project by reading wikis or forums or be part of mailing lists. They simply use the OS, they base their trust on it that it WILL work.

So, I ask again. Now that the damage is done (and I am sure I won't be the only one), how do I fix this?

If this is not fixable and requires me to delete the /opt/kde/ folder then this is a major error of the Arch Linux administration, because this will mean that the user will have to lose data (I have some third party kde apps installed there).

ah, ok, thanks, will do. So, the second time I will re-install kde 3.4 it will work.

oh and i also did a comment while installing that is linked to the wiki, so every user is informed of the things what might happen.