Trivia - what is this plane's primary function?

Boeing Co N378BC, originally uploaded by Drewski2112.

Give up?

Let's start w/ the obvious. Yes - it is a Boeing 737. Yes - it is painted for the RAAF (Australian Air Force).

But - what is the purpose of the extra 'fin' running down the center of the fuselage?

That is a Multi-role Electronically Scanned Array (MESA) - because this is a AEW&C aircraft - or more simply - AWACS. With the new Electronically scanned array - think Aegis in the Navy - you no longer need the flying radome on top of the USAF E-3's or USN E-2C's.

Basically - this is a 737-700 (equivalent to the BBJ) - w/ extra equipment for a modern airforce. Australia has ordered 6, Turkey is buying 6 and South Korea is looking at this as well.

As my brother correctly pointed out - the 737 AWACS falls under the 'Wedgetail' project name.

(More info on the 737 AEW&C project)

If you want to see more pictures from the Seattle area airports - including Boeing Field and Renton Muni (where the 737 is assembled) - then take a look at Drewski2112's pictures over on Flickr.