There are problems with ACPI management on my HP nx7400 laptop. The laptop does not detect changes in power source (for example, when AC adapter added/removed). There is also some weird fan behavior. Furthermore, it does not wake up from suspend2ram - the screen remains blank and keyboard does not respond to commands.
This is a common problem on all recent HP nc/nx series laptops. These laptops go into 'bad state' on system shutdown/restart. When in 'bad state' the before mentioned problems appear, but there are also reports about some more nasty things - some laptops hang up to 20 seconds on startup/booting from BIOS. One workaround is to remove 'psmouse' module and 'i8042' module before switching off/restarting laptop.
The bug is appears to be fixed in 2.6.21-rc2 kernel version. See this link for more information: http://bugzilla.kernel.org/show_bug.cgi?id=7689. More information about the bug is also included here: http://<email address hidden>/msg05126.html
Since this bug affects relatively large number of HP laptop users and can have nasty consequences (CPU overheating because the fan is not working), I suggest to include backported patch in Ubuntu kernels.
Feisty Beta does not have this problem. I checked Ubuntu kernel sources, it seems that the beforementioned fix is included in them. Some ACPI problems remain with suspend2ram functionality (laptop wakes up with black screen / without mouse and keyboard) but that's another bug.