A host of blue-chip companies have altered their entries on Wikipedia, the online encyclopedia, in an attempt to cover up embarrassing episodes in their history.The discovery was made by WikiScanner, a site that traces the source of changes to the world's largest online reference work by matching edits - which, famously, can be made by anyone - to a database of the unique "IP addresses" of the computers that were used to make them.Machines belonging to organizations including Wal-Mart, Disney, Sony, the Labour Party, the CIA and the Vatican, have been used to rewrite entries, it alleges. WikiScanner was developed by Virgil Griffith, 24, a researcher at the California Institute of Technology.