First of all, what is it? It's the idea putting your manuscript (or a CD or something) in an envelope and mailing it to yourself, so you have a date -- from an external source, not your own computer's clock which you could, of course change -- that hypothetically proves you were in possession of that document as of a particular day.
While I can't speak for laws in every country (and I'm not an attorney so I can't give legal advice anyway), the consensus online seems to be that there's no point to this. You don't get anything from it. You do lose the money you spent on paper, the envelope, and postage.
Here are some reasons you shouldn't bother:
- It can easily be faked. This website lists some examples.
- No one has demonstrated with a specific example of how it worked for them. (See the reference linked to in the above bullet point.) The reports online have all the veracity of e-mail forwards about new attempts by criminals to con people (you know the ones: oh god, that perfume sample sprayed on that card that the strange man in the parking lot of Big Lots/Walmart/some other shitty store asks you to smell is really ether/chloroform/GHB and it will knock you out and you'll get robbed/raped/kidnapped). In fact, Snopes.com has an entry for PMC.
- It doesn't give you standing to sue in federal court. (Click here for details.)
When to watch out: if you are going to self-publish, especially in an electronic format, you really might want to pay the fees at the copyright office. (Hell, include that on your Kickstarter or something.) Because there have been many cases of people harvesting work online and republishing it on Amazon under fake names, then raking in cash on the Kindle store until Amazon finds them and suspends their accounts. You'd basically have no recourse if you hadn't paid the registration fees. Although I wouldn't recommend self-publishing, but that's a separate issue.