I can confirm this is a problem on my HP Compaq nc6230. Intel Pentiume M processor 1.86Ghz. Running the official kernel 2.6.15-25-686 gives me a couple dozen percentage points higher CPU usge when idle while the 2.6.15-25-386 kernel leaves me at near 0 percent CPU usage when idling. It's always the X process that shows to be using the CPU time. Here is the process as listed in gnome-system-monitor:
/usr/bin/X :0 -br -audit 0 -auth /var/lib/gdm/:0.Xauth -nolisten tcp vt7
gnome-system-monitor shows overall CPU usage at ~50% while top shows a fluxtuation of ~15-35% (it's harder to see what the real usage is w/o a graph). The 386 kernel usage is what should be expected, which is nearly at the bottom for CPU usage. All of this is while "idle", of course.