For years I always approached it as a decending 2/5 progression....but lately I've reveresed my way of thinking and now approach it as a diatonic progression (although this goes against all of theory). But when you think about it the turnaround progression is there to outline the 1 chord, and when you hear the progression used in a song it sounds pretty diatonic. Plus I find I play much more interesting lines this way, were as if I'm approaching it as a descsending 2/5 I tend to play patterns.
This is a great question to throw at a jazz player cause you will always get a different answer each time.

Hey T.D...if you get a chance ask Jimmy the scale he uses over Giant Steps (he made up a scale that works over the 1st 8 bars of G.S...I used to know it, but have since forgotten it. I remember it started on B and was either half major/half diminished or vice versa).