Widebody; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Conflict NMAC
Flight Crew Took Evasive Action
Human Factors; Procedure
We were on a vector for 13R approach to BFI; configured Flaps 1; speed 190 KTS with SEA approach when an unidentified aircraft appeared on TCAS 2.5 miles ahead.  Approach had just given us a vector towards this 'pop-up TCAS target at our altitude' and we queried Approach if they were aware of the traffic.  At that moment an unidentified voice on the radio said; 'I just turned on my transponder.'  At that moment TCAS advised to monitor vertical speed then told us to 'Climb; Climb' and the Co-Pilot immediately disengaged the automation and followed the TCAS VSI out of the red arc while simultaneously applying full power.  I immediately notified ATC we were in a TCAS climb.  After passing over the aircraft at approximately 100-200 FT (according to the TCAS readout); TCAS announced we were 'Clear of Conflict' and we advised ATC who re-cleared us to our original vector and altitude for the approach.
Large transport aircraft being vectored for approach to BFI; responded to a TCAS RA and experienced an NMAC with 'pop-up' traffic that had just activated the aircraft's transponder.