I'm a little late getting to this one, but the Baltimore Museum of Art has prevailed on summary judgment in a custody battle over a Renoir stolen from the museum in 1951. Patricia Cohen has the story here.
UPDATE: Fox Rothschild comments: "This case highlights that provenance is key! ... All buyers of fine are advised to gather as much
documentary evidence from the seller at the time of purchase and to keep
receipts and any other documentation to show when, where and how the
item was acquired in case a dispute over title arises."