I use some applications (konversation, kontact through kmail icon, ktorrent) and minimize them in the system tray. What I am missing is a shortcut to bring one of those applications back to the front. Is it implemented? Where?

I believe you will have to wait until KDE4.3 - end of July. Until then the mouse will have to be your friend.

The future is now, er... I am using KDE 4.trunk/

Alt+Tab->Expose windows, Control+Alt+/->switch current screen, all those work great. The missing bit is returning to those tray'd applications.

You should be able to configure System Settings > Input Actions with the relevant Keyboard shortcut and DBus call to show the application.

I did not see input actions. Yes, that's it, almost! So, for Konversation is:
org.kde.konversation /konversation/MainWindow_1 show

It does open the window, but behind the current one, maybe others too. It would be great to have the system tray icon raise action available too. Or, if I am missing another DBus call, please tell me.

You could try enabling some Window rules by right clicking on the window decoration, which lists access to several configuration dialogs.

I did not like forcing the window to be above all, so I disabled focus prevention in general and it works. But I still think both methods are workarounds. I have been considering doing a bug report like this:

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Bug report deleted by the author.
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Disabling focus prevention per application is fine. Solved! Thanks.