We thank Alan Newell for the feedback he gave us about our results during his RPI visit in March 2004, particularly for pointing out that a very similar PDF equation was in derived in Prigogine's book.
We also thank Colm Connaughton for his comments and for the proofreading of the manuscript.
Yeontaek Choi's work is supported by KOSEF M07-2003-000-10003-0. Yuri Lvov acknowledges support provided by NSF CAREER grant DMS 0134955 and by ONR YIP grant N000140210528.