Air carrier departure from ATL described confused takeoff event; when similar sounding call signs resulted in a questionable ATC clearance and aircraft runway entry; event involved same air carrier aircraft with similar four digit flight numbers.

Narrative

We contacted ground control in ATL at the north side of the ramp with 'air carrier XWZZ1' as the call sign. Ground control gave us clearance using 'air carrier XWZZ1' to taxi to Runway 8R via echo behind an MD-88. Then furthermore telling us we were in sequence behind the MD-88 and over to tower. We were cleared into position and hold on Runway 8R under 'air carrier XYZZ2' but replied with 'air carrier XWZZ1.' After starting onto the runway we started to question the clearance with each other but had already started moving past the hold short line. We got into position and cleared for takeoff under the same wrong call sign and then queried the Controller at which time they cancelled takeoff clearance. At that time company came on the radio saying they were 5 back in line for takeoff and it seems they switched the departure clearances slips. The Controller quickly pulled the correct clearance out for us and we went on our way. The mistake took only seconds to correct but looking back we should not have entered the runway unless we heard the correct call sign. Looking back I thought it was a simple mistake and they just misread our call sign and we were helping out by keeping the flow of departures moving in ATL. I understand to not enter the runway under a wrong call sign and plan to question as soon as possible if this happens again. I also understand holding up traffic in ATL while a mistake like is cleared up would have been brief and I should have queried sooner.