With the opensource driver, ATI Radeon cards run very hot and the fan runs continuously, resulting in a sizzling laptop and short battery life.  Many users switch to the proprietary drivers to avoid the problems.

By default, the power profile is high power for Radeon cards.  A better default (which gives similar behavior to the proprietary driver) would be to set the power profile to auto, which causes good performance when on power, and low, cooler performance when on battery.

To do this, I put the following in /etc/rc.local:
echo auto > /sys/class/drm/card-0/device/power_profile

There are heaps of mentions of this from users in forums around the place, and a bit of info here:
http://www.overclock.net/linux-unix/731469-how-power-saving-radeon-driver.html