Most recent kernel where this bug did not occur: 2.6.15.1
Distribution: Ubuntu Dapper
Hardware Environment: HP/Compaq nc6120, Intel Centrino/Pentium-M
Software Environment: Ubuntu/gnome
Problem Description:

Since the 2.6.17.1 kernel, acpi sometimes doesn't correctly detect the power
status anymore: the battery life is stuck at a fixed (but seemingly random)
time, and plugging in the power plug is not detected (even though the laptop
actually keeps running with the power connected and the battery removed).

After a fresh reboot or suspend/resume cycle, everything returns to normal (the
battery life and presence of external power are detected correctly), but after a
while (several hours) it stops functioning.  The /proc/acpi/battery/C16D/state
then is fixed to something like

present:                 yes
capacity state:          ok
charging state:          discharging
present rate:            1829 mA
remaining capacity:      1720 mAh
present voltage:         10906 mV

(with different numbers on each occurrence), even though the leds on the laptop
show that the external power is connected and the battery is charging.  The ac 
power state in /proc/acpi/ac_adapter/C16B/state simply says that the power is
off-line, regardless whether the power is connected or not.