The Desert Island Test is a CornerCase example often used by participants of the Debian-Legal mailing list to define what qualifies as FreeSoftware. The Desert Island Test emphasizes the freedom to distribute FreeSoftware without the consent or knowlege of a third party.
The example was clearly defined by ?ThomasBushnell in the following paragraph.
"A good test case for whether a license is free (for issues like this) is whether a disconnected group of people on a desert island could distribute the software among themselves. In the vim case, they cannot. (For example, if the vim maintainer flies over the island and drops down a message saying "you must hereby send me your changes", how are the people down below to comply?) The fact that the vim maintainer can send the request does not say anything about whether the people receiving it could reply."
The Debian-Legal mailing list: http://lists.debian.org/debian-legal/
The desert island test: http://lists.debian.org/debian-legal/2002/01/msg00010.html