Originally Posted by DeafFlyer
Right, with the only exception being New York City. Before anybody says it, some jurisdictions, like Washington DC have limitations on use of out-of-state placards but they are still valid. (In DC you can't park at a parking meter for free or for twice the meter's time, like those with DC Placards can. You can park in designated spaces with out of state placards though.)
I have read the signage in DC. Anybody who went to court to challenge this under the Full Faith and Credit clause would win.