Ha! Interesting question. I just ran an all state/federal Westlaw search for "IANAL" and didn't hit any case law. Two or three briefs, a couple of scholarly articles, but no court opinions. I also searched for "I am not a lawyer", but that brought up about 130 caselaw hits which seemed to primarily be references to various filings by pro se parties actually saying in a court filing "I am not a lawyer".