An Air Carrier First Officer described a track deviation which resulted when both pilots diverted their attention to programming their respective EFB's during a DFW arrival.

Narrative

I was the pilot monitoring as we approached DFW on the BYP5 Arrival. Just after passing the KARLA waypoint at 11;000 FT and 250 KTS as assigned approach advised us of our landing runway. The Captain loaded the ILS to Runway 17L into the FMS and activated the flight plan. I believe we were both loading the approach into our EFBs when the Captain noticed that the airplane was in a left hand turn back towards LIT which was the initial approach fix on the arrival. He immediately intervened and began a turn back to our assigned course. (We were approximately 50 degrees and 1 to 2 miles off course.) At this time ATC told us to turn right to a heading of 240 degrees vectors for the ILS Runway 17L. He then asked us where we were going? It seemed as though the Controller didn't notice our course deviation until after he had assigned us the 240 degree heading. The Captain responded to the Controller's question by stating that we had a problem with our FMS but we were now on our assigned heading. The Controller made no further comments relating to the deviation and the flight continued normally. After the landing runway was loaded into the flight plan both pilots should have confirmed that the correct runway was loaded and that the aircraft was still going to continue to the next waypoint in sequence on the arrival. After the flight plan was activated both pilots should have monitored the route the airplane was taking and intervened immediately when the aircraft began to turn off course.