Funny I didn’t think about that.

It is commonly believed by non-lawyers that it is impossible to put a work into the public domain. Although copyright law generally does not provide any statutory means to “abandon” copyright so that a work can enter the public domain, this does not mean that it is impossible or even difficult, only that the law is somewhat unclear. Congress may not have felt it necessary to codify this part of the law, because abandoning property (like a tract of land) to the public domain has traditionally been a matter of common law, rather than statute. (Alternatively, because copyright has traditionally been seen as a valuable right, one which required registration to achieve, it would not have made sense to contemplate someone abandoning it in 1976 and 1988.)

And then later on:

This decision holds that computer software may enter the public domain through “freely accessible program exchanges and the like,” or by becoming “commonplace in the computer industry.” Relying only on this decision, it is unclear whether an author can dedicate his work to the public domain simply by labeling it as such, or whether dedication to the public domain requires widespread dissemination.

It sounds like there isn’t a clear way to mark something as public domain. On the other hand, it’s pretty trivial to attach a very permissive license, such as the MIT license, to a piece of code. But once again, I’m not a lawyer, so I could very well be wrong.