Answer:
For most practical purposes, it is — sort of. This is a complicated question, so please read on.
"Public domain" is a technical term in copyright law that refers to works not under copyright — either because they were never in copyright to begin with (for example, works authored by U.S. government employees, on government time and as part of their job, are automatically in the public domain), or because their copyright term has finally lapsed and they have "fallen into" the public domain.
Not all jurisdictions have a public domain, and it doesn't always mean exactly the same thing in the jurisdictions that do have it. Furthermore, even where it is clear what it means, it's still not a license. To be subject to a license, a work must still be in copyright. That means there is no way for the "public domain", as a concept, to go through the OSI evaluation and approval process. We wouldn't be evaluating a license text. Instead, we would have to somehow evaluate the laws themselves, in different jurisdictions, and say which jurisdictions have a public domain that meets the Open Source Definition and does not create problems for software authors and users. This would be very difficult, because it would mean evaluating not just the statutes but various bodies of case law (for example, open source licenses usually have a strong disclaimer of liability for the copyright holder — but we don't know how or whether the author would be protected from liability for software released into the public domain in various jurisdictions). This approach would not be useful to the OSI's mission, because open source is an international phenomenon and we only want to approve licenses that meet the Open Source Definition everywhere.
Thus we recommend that you always apply an approved Open Source license to software you are releasing, rather than try to waive copyright altogether. Using a clear, recognized Open Source license actually makes it easier for others to know that your software meets the Open Source Definition. It also enables the protection of attribution, and various other non-restrictive rights, that cannot be reliably enforced when there is no license.
There are certain circumstances, such as with U.S. government works as described above, where it is not easy to apply a license, and the software must be released into the public domain. In these cases, while it would be inaccurate to display the OSI logo or say that the license is OSI-approved (since there is no license), nevertheless we think it is accurate to say that such software is effectively open source, or open source for most practical purposes, even though it is not officially released under an open source license. (This is assuming, of course, that in the laws of releasing jurisdiction the meaning of "public domain" is compatible with the Open Source Definition.) After all, the freedoms guaranteed by open source licenses are still present, and it is possible for the familiar dynamics of open source collaboration to arise around the software.