Adventure Developer has finally run the much talked about (in some circles) opinion/advice column, "A lesson on Legal Anarchy: How to Avoid the Cold Wrath of LEC." The article, by The Adventure Game's "Yorik Kingzjester esq," discusses the do's and don'ts when making your LucasArts-themed fan game or addon.
The article deals in US copyright law and common sense, pulling examples from Narrative's Legends of LeChuck, and Scurvyliver's Fate of Monkey Island, one of the first fangames to be shut down, as well as others. I, um, haven't read the whole thing yet, but probably should.