There have been numerous anecdotal stories about companies or individuals editing their Wikipedia listings, typically in hopes of removing negative information. In other cases, such as that of a campaign manager of a Georgia Gubernatorial candidate, the effort was to disparage a competitor.
Of course, in virtually all cases, other Wikipedia users will quickly notice the vandalism and restore the original page. But that doesn't keep people from trying.
Now, there's an easy way to identify potential whitewashing. CalTech student Virgil Griffith has created the Wikipedia Scanner. The application cross-references listings with the IP address of those who make the edits.
In this Wired article, they describe how it has uncovered cases such as voting machine manufacturer Diebold trying to delete numerous paragraphs about their CEO's relationship to President Bush and WalMart's editing text about its wage policies. Some of the cases seem more innocuous, albeit strange, such as some user at the CIA changing the lyrics of a song on the show Buffy the Vampire Slayer (you'd think they have more important things to focus on).
By making the database publicly available, Griffith is encouraging users to do their own searches and reveal potential whitewashing efforts. Or, just click on some of the examples on the Wikipedia Scanner page. The results are interesting. For example, while the NY Times has 1,285 edits, as compared to just 102 for the Fox News Channel, most of the Times entries are focused on external news topics, while most of the Fox News edits are to their own staff, such as Brit Hume and Chris Wallace or to their nemesis, Keith Olbermann of MSNBC.