Never trust another computational agent unless you can see its source code?

Never trust anyone unless you're talking in person? :p

Never trust other thinking beings if you don't know the location of their intelligence center so that you can destroy it if necessary?

Talking to Clippy? As in, I don't.

Telemarketers.

The reasonable way to interpret this seems to be "don't trust something you don't understand/cannot predict." Not sure how seeing where it keeps its brain helps with that, though.

Actually my first thought upon reading that was "follow the improbability" -- be suspicious of elements of your world-model that seem particularly well optimized in some direction if you can't see the source of the optimization pressure.

This is the allusion I had in mind, but actually I've had occasion to quote this when talking about corporations and similar institutions. If an organization doesn't keep its brain inside a human skull (and I'm sure some do), it seems guaranteed to make bizarre decisions. Anthropomorphizing corporations can be a dangerous mistake (certainly has been for me more than once).

A much more concrete example is cloud computing. Granted, computers don't "think," but it's a close enough analogy.

You must always keep in mind that there is no magic "cloud"- only concrete machines that other people own and keep hidden from you. People who might have very different ideas than you on such matters, as for example, privacy rights.