Trespassing is not the same as going somewhere you weren't invited. It is always wrong for journalists to trespass (i.e. on private propery), but it's not wrong for them to show up uninvited.
Curious where the line is, i.e. if Demo Days were at YC's offices (which it used to be, right?) would it be trespassing to show up uninvited? It's private property after all.
IANAL but if there's no locked doors or signs saying you aren't welcome to an apparently public place, then I don't think it's trespassing until someone asks you to leave. A shopping mall is private property too, for example.