You may need to upgrade to KDE 4.3 ( which will be included in openSUSE 11.2, and is available from the openSUSE BuildService also )

If you access the clock's settings, then change to the Time Zones tab, you can use the shift key to select multiple time zones, and select the default one it shows you when you are not hovering over it.

I have now bought a new computer and upgraded to openSUSE 11.2 and KDE 4.4.3. The clock now behaves as expected - I can add multiple timezones again.