Bug#746589: More on this bug
I have it too, rendering a new 64-bit wheezy 7.5 install unusable!
I doubt that kdm itself is the problem though I tried to file a bug for this a well. The 4.8 version with all the 4.8 packages worked flawlessly. It was some other upgraded package, most likely not part of KDE that killed it. I upgraded the kdm and a lot of kde stuff as well to 4.11+ in desperation to get it working again, to no avail.
There is a similar 2011 bug still around against dbus #649362
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