Computer locks instead of hibernating when battery is critically low

Binary package hint: devicekit-power

On Lucid, on an EEE PC 901, I waited for the battery to go critical. I first observed warnings saying that battery is low, with options "Ok" and "Cancel". What do these buttons do? Why is this not done through notify-osd? The warnings also popped under my terminal window, so I almost missed them.
After the warnings, I got the message saying that battery is critical and that the computer will hibernate unless plugged in, with similar "Ok" and "Cancel" options. The computer then blanked the screen, and presented me with the unlock screen with the password field. I expected the computer to hibernate, not to just lock the screen.
Note that I use uswsusp for hibernating, and the usual pressing the power button and choosing hibernate from the menu works; only this hibernate when battery is critical does not. pm-hibernate also works.