Bug#528682: found the problem, not X11 related
After installing 2.6.30 without discovering new behavior, I realized that
my CDROM apparently causes IDE bus resets from time to time. Removing the CDROM
from the system lets the laptop get past the kdm login prompt. My suspicion is
that kdm/KDE/... does some initialization thing that triggers some combination
of kernel behavior and hardware issue, such that the kernel finally stalls.
Sorry for the confusion and thanks for your suggestions/help.
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