I would believe the Iroquois were admired for making a Constitution. Its still a stretch though, to say something was copied.
If anyone wants to judge for themselves:
http://tuscaroras.com/pages/history/iroquois_constitution_1.html
I imagine words like "inspiration" and "admiration" are more suitable.
wikipedia entry suggest there's a good bit of debate about it.
http://en.wikipedia.org/wiki/Iroquois_Confederacy#Influence_on_the_United_States