This started off as a reply to Sleestak's article The Wolverine Conspiracy over at Lady, That's my Skull, but it got a bit too long so I decided to post it here instead. Sorry, Slee.
Sleestak discusses what qualifies as a cameo in a comic book. I looked up "cameo" in the dictionary and it wasn't really specific enough, merely suggesting that it is the brief appearance of a prominent actor in one scene of a movie.
In comic terms a cameo is much less; usually a single panel appearance or standing in the background for a couple of panels as furniture.
The problem with comics is that they are formed of both single self-contained stories and
multi-part or continued stories. Sometimes at the same time. In the case of Hulk #180 - 181 Wolverine's appearance only counts as a cameo if you take it as self-contained, which it clearly isn't. Read the story as a whole and the end of the comic is not the end of the scene. 181 continues on from the end of 180 continuing the scene unbroken. Therefore Wolverine's appearance in 180 cannot be a cameo when taken in context.
I think a fairly adequate rule of thumb would be if you can remove a character from the story without it having any impact at all on that story then it's a cameo. I'd include "furniture" appearances in this - those shots like crowd scenes in Crisis with a lot of big name characters standing around: it is only important to have the crowd. It doesn't matter who is specifically in the scene.