And even if they were that silly, I'd be pretty confident that a fair use defense (parody) would hold up here, if something so ridiculous were actually to go to court.

By that logic everyone that uses Helvetica in Black on a white background should be sued by American Apparel.

As far as I know, trademarks can cover the proportions of the logo. So, if I created an orange square, put a lambda in it (using the same font) and ensured the padding around the lambda was pinpoint accurate, that'd totally be infringement.

About the only thing that I'm being sketchy on is the use of the Y Combinator name. I tried to use Y-Combinator, when referring to math, and Y Combinator when referring to the company, but the line is a bit blurry...