Distribution/Version: Ubuntu 5.04

The AC adapter state can change without ACPI event in the following method (at
least on my laptop):
-Initial state: plugged in
-Put laptop to sleep (or suspend to disk)
-Unplug AC adapter
-Resume laptop.
(or vice-versa, plugging laptop in during sleep if it was originally not does
same thing)
The battstat applet will continue for eternity to have the icon suggest that AC
power is connected.

Here is a quick patch I used for my purposes:
--- gnome-applets-2.10.1/battstat/acpi-linux.c  2005-01-13 23:06:40.000000000 -0500
+++ gnome-applets-2.10.1-patch/battstat/acpi-linux.c    2005-07-01
22:55:32.000000000 -0400
@@ -374,6 +375,8 @@
   /* Get the remaining capacity for the batteries.  Other information
    * such as AC state and battery max capacity are read only when they
    * change using acpi_process_event(). */
+  /* State changes during sleep do not have associated events */
+  update_ac_info(acpiinfo);
   while ((procdirentry=readdir(procdir)))
    {
     if (procdirentry->d_name[0]!='.')

There might be another approach more elegant, i.e. being notified explicitly of
resume, however, no ACPI event accompanies it that I can tell and other
solutions would require sleep/resume scripts to pass data to battstat, which
varies between distros.  This approach checks status of ac adapter every battery
update, which means it doesn't update immediately on resume, but at least it is
better than the current behavior.