Portland Judge Finds Fault With Fixies
"On June 1, 2006 Portland bike messenger Ayla Holland was given a ticket for allegedly violating Oregon Revised Statute... Holland has 30 days to either attach a hand brake to her bike and pay a $73 fine, or appeal the decision."
Source: http://bikeportland.org/2006/07/28/judge-finds-fault-with-fixies/
Seattle Bike Code contains similar phrasing:
Section 11.44.200 BRAKES. Every bicycle shall be equipped with
a brake which will enable the operator to make the braked wheels skid
on dry, level, clean pavement.