There's an interesting documentary called "The Man Who Forged America" about a guy named Mark Hoffman who was a master forger. It's not about sports memorabilia at all, but it's worth watching because it shows a number of forgery techniques he used that are definitely relevant to a number of the things people here might collect. His story also has a crazy and unfortunate ending after he gets caught in his web of lies...
http://www.megavideo.com/?v=YORKKIAT
(hit play, close pop-up window, hit play again)