By the end of 2012, some US state will have used a system of proportional representation to elect
Under proportional representation (for purposes of this claim), representatives are elected from multi-seat districts in proportion to the number of votes received.
I would include as Proportional Representation:
1) STV 2) Party List PR 3) Mixed-Member PR
(these are all traditional PR methods)
In addition, I would accept systems that allow Proxy voting or election of a legislature by lottery as Proportional representation.