When I say "cameo", I mean he's not one of the central characters in the book.
I'm re-reading Going Postal at the moment but don't recall him appearing in the book, whereas I do remember Carrot and Angua appearing. But I could be wrong, as usual.
WARNING! CONTROVESY AHEAD!
As for "Where's my cow?", I could forget that very easily, because I wasn't very impressed by it. If you'll excuse the puns, I thought it was a "cash cow", designed to "milk" Discworld fans.
I would also class Discworld stamps, the Unseen University leather bound editions, and probably (but I hope I'm wrong) the forthcoming football trading cards, as similar. Fair enough, because Discworld is a business designed to make money and if people want to buy these items, good luck to them. I personally would like Sir Pterry to earn as much money as he possible can, but it doesn't mean I approve of, or would buy everything Discworld-ly.
(waits to be shot down in flames and burnt as a heretic