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From spec point of view, JSF 2.1 is a maintenance release 2 from JSF 2.0, so any application written for JSF 2.0 will work with 2.1 jars.
There are no major changes between those versions than can cause you incompatibilities. By that reason, in practice both lines (2.0 and 2.1) are maintained by MyFaces Community at the same time.