This was covered in depth recently by the Bluecoat forum (Bill Bulfer's 737 internet site)- I can't remeber the address off hand but I'm sure it's easy to find.
Anyway, the official Boeing answer was that that amount of fuel was deemed the minimum necessary to complete the checklist and land safely. Any less than that and it was consideder better to not enter into the lengthly checklist when the time could be better spent preparing the a/c for landing.
Oh, and it's found in the Boeing Master Non normal checklist - not your average on board version. It's the kind of advice that would be passed onto you from ground based engineers.
Hope this helps!
[This message has been edited by Sink Rate (edited 27 November 2000).]