B737 Captain describes the circumstances leading up to a runway incursion at Taxiway W and Runway 10C at PIT.

Narrative

ATIS information was departures expect 10R & 10L. Previously briefed taxi to Runway 10R as via Delta 2; Echo; Whiskey; depart Runway 10R. During initial contact between Ramp; Ground; and with the First Officer heads down with W&B; ground changed clearance to 'expect 10C'. I didn't hear the change. While turning left onto taxiway Whiskey headed toward 10R First Officer pointed out 10C as new departure runway. Our nose was almost breaking the plane of the 'chevron' area which is the extended area west of 10C. Tower cleared us for takeoff. We saw no Runway 10C signs (or hold short signs). The Tower made no mention of our position. For all I know; from his vantage point; we may have appeared to be holding short of the extended approach area for 10C. I'm not submitting this ASAP in concern that I taxied somewhere I wasn't supposed to. I'm more concerned that the west end of 10C/28C is not very well marked. After extensive review I still can't tell you whether it is permissible to taxi south on Whiskey to 10R without getting clearance to cross the west 'extended non-operating chevron surface' of 10C. I think there should be more runway markings to clear this up like flashing 10C hold short lights. But; on the other hand; if it is acceptable to simply taxi south on Whiskey toward 10R without a runway 10C crossing clearance then that should be spelled out as well. My First Officer and I found this area to be confusing.