On Asus K53 most of the hot-keys are not working with the default installation of Ubuntu 11.04 with kernel 2.6.38-8, however there is a package available that will enable them, here are instructions on how to perform that:

sudo apt-get install git  (unless you already have git installed)
git clone git://git.iksaif.net/acpi4asus-dkms.git
cd acpi4asus-dkms 
make 
sudo make install
sudo modprobe asus-nb-wmi   (this command will load the newly installed module)

After every kernel update the above will be erased and you will need to re-do it every time until it is included in a kernel version at a later stage. To make it easier create a .deb package which will be easier to install.

sudo apt-get install debhelper 
cd acpi4asus-dkms 
dpkg-buildpackage
sudo dpkg -i [newly-created-package].deb
sudo modprobe asus-nb-wmi