This is a weird one and I can't seem to get it to reliably replicate...
I'm on an HP Pavilion dv4 laptop running Mint 8 Gloria GNOME edition and my power management randomly ignores the AC adapter status. The computer will work fine for a while, then after a few rounds of closing/opening the lid and plugging in and unplugging the AC adapter it suddenly can't detect any more changes in the power source, but other ACPI events such as manually putting the machine to sleep, pressing the power button, and the lid button still work.
The problem never resolves and I end up rebooting to reset it, where it will work for a time then start ignoring the AC status again.
I attached a copy of my gnome-power-bugreport and lspci outputs if they're any help.