I suppose that technically, by definition, having an open relationship might constitute sexual infidelity. However, I feel that if both parties in the open relationship have desired and accept this type of union, then it's not really considered being unfaithful.
To me, infidelity is marked by a betrayal of one or both partners against the other. if both parties consent to such an open union, the betrayal would not be there.
Many open partners put certain limitations on their "openness." For example, you can't sleep with the same person more than 1-3 times. This helps avoid having a parallel "relationship" that might threaten the primary relationship. Or perhaps not sleeping with someone that is inside of one's social circle (i.e. no friends, in-laws, etc.) I think that ignoring those limitations would constitute infidelity.