After programming their LAS departure via the SHEAD RNAV SID from Runway 25R; the A320 flight crew instead departed via Runway 1R. When 1R was entered into the FMGC the departure procedure failed to transition to the revised track.

Narrative

During preflight while programming the box; we originally set 25R. Later; with an updated ATIS; we changed the runway to 1R. We verified the departure for the Before Start checklist; then again during the Before TO checklist. As First Officer and pilot not flying; I verified during the FMGC check by double clutching 1L on the FMGC and read 'Runway 1R; Shead; OAL transition'. During the takeoff roll; I visually verified 1R on the Navigation Dispaly. Shortly after takeoff the lateral command bar turned left; instead of waiting until the 2;681 FT bubble; which was displayed; which would have taken us into the strip. The Captain ignored the command bars until 2;681; then started his left turn. Then; the next fix was RBELL; instead of BESSY. The box thought we were taking off from 25R. We told ATC that we were having navigation problems and flew an assigned heading of 210. A few moments later; after I manually inserted the correct fixes; we were given a turn direct SHEAD. At no time were we more than 3 miles off course and passed between the two fixes; RBELL and BESSY.