This is a current problem for Dapper, Breezy and Hoary whatever desktop you are using (KDE, GNOME or XFCE).
On some hardware – like my Dell Latitude D600 – you get constant disk access and noise. This does not allow your disk to spin down to save energy (especially annoying when running on battery). This problem is reported in this forum: Frequent HDD activity
So far I have manage to activate the laptop mode, but it does not seem to solve the problem for me. Anyway, if you want to activate laptop mode this is not straight forward, and this is why I am writing today.
You first need to modify the /etc/default/acpi-support file to allow laptop mode: ENABLE_LAPTOP_MODE=true
Then you need to update /etc/laptop-mode/laptop-mode.conf to use laptop mode even on AC power. ENABLE_LAPTOP_MODE_ON_AC=1
Finally run /etc/init.d/laptop_mode restart and you’re done.