An in-joke, an hommage or a copyright infringment?

Anybody knows what’s the legal situation with humorous nods and in-jokes towards other games and universes? I mean…

… I can spot several items here which are definitely not part of the Space Quest universe. I can’t imagine they went out and asked for permission on each and every one of them, though.

Putting the doctor’s T.A.R.D.I.S in a backalley of your adventure game set in London? The shape of the Millennium Falcon on a landing pad in the background of your space sim? Is there any line to draw here?

For a question in that magnitude of copyrighted material you better ask a lawyer specialized in media/IP. You will not get any legally binding answers on a public forum. The only thing in such a situation that is always right: If you can’t afford a lawyer or do not want to make big money with your game: do not put copyrighted things you are uncertain of in there and stay away from any material you have no license for.