I believe what you migth be looking at are the "cliff sensors". They will detect if the floor from underneath the roomba vanishes - and thus it will stop to avoid driving down some stairs or over a big dropoff.
I don't know if you could utilize these sensors to follow a line unless it was driving over a very "controlled" surface. You probably would have the best chance at utilizing the sensors to do this by using alumium tape for the best reflectivity, and driving the roomba on a rug (very un-reflective).
The roomba itlsef is not very smart however, so you'd have to feed the sensor feedback into arduino, and maybe power the emitters with a lower voltage level. I assume that since the leds are so close - they might be "saturating" the reciever no matter if the surface is dull / diffuse.
If you look at teh setup of the sensors, they are set at a specific angle - so really the sensors are more setup to work to complete a "light circuit" just based on geometry. When there is no floor for the light to reflect on - the reciever will no longer pick up the reflected light - and detect the cliff.
Check this out / this will probably give you some info on it:
http://www.robotreviews.com/chat/viewtopic.php?p=96901
Hope this helps, and if you have any further questions about this, programming or electronics, ask away!