Fall protection systems for ladders are only required for ladders more than 15 feet tall (you can see the harness attachment rail at the center line of this one). This may keep you from falling to the ground, but you may still bust your face on the ladder before you regain you composure (yet another good reason to have a chin strap on your hard-hat!).
Many of the ladders you may encounter are right at or just short of the 15 foot limit, so all you have to protect you is your good grip on the ladder. You can install fall arrest devices on ladders less than 15', too, and it is a really good practice.
Safeguard Technologies has a solution: Formed fiberglass overlays that are fabricated to fit a variety of ladder rung diameters from 3/4" to 2" diameter. The covers have hi-traction grit embedded in the material to provide a slip-resistant texture to the surface. These can be applied to scaffold climbing rails, too, which are notoriously slippery - why do the scaffolding manufacturers coat these with glossy powder-coat paint?!
Resource: Safeguard Technology (http://www.safeguard-technology.com/)