I noticed in the keyboard settings (K -> System Settings -> Keyboard & Mouse -> Keyboard Shortcuts) that Alt + LeftArrow is mapped to Back in the Application Shortcuts, but it didn't work for me.

Then I started playing around...

Right Alt + Arrow Keys just acts like the bare Arrow Keys.

Left Alt + Left Arrow Key acts as I expect - it is the same as clicking the Back Arrow key in Firefox or Konqueror. Similarly for the Right Arrow key.

My guess is the different scan codes aren't being mapped to the same action. There doesn't seem to be an obvious setting for it in the Keyboard Shortcuts.

Cheers,
Scott.