I've wondered before - what's the difference between WTFPL and just releasing something into the public domain? The name-change bit seems to relate to the license itself, rather than the licensed work.
The problem is that there is no such thing as a universally agreed upon concept of public domain. If you actually care about licensing and want your project to globally and legally be in something like the public domain, in the US sense, then you're going to have to use something like the creative commons CC0 license.
Of course if you don't really care all that much about the details of international copyright law and just think it's cool to swear on the internet then the WTFPL will be fine.
The thing is, there is no universal notion of public domain. For instance, you could say that a work in the public domain belongs to no-one and/or everyone... but in France (and, as I juste learned, in the rest of Europe as well [0]) there is this thing that says that the creator of a work will always have paternity on it, forever.
I think most people have the same notion though, but as you know law is not just about notions, it's about hardcoding what's ok and what's not. This is actually what the CC0 tries to "solve": if there is no notion of public domain in the country you live in, then you apply the terms of the license, which happen to be what "public domain" would be.
Of course if you don't really care all that much about the details of international copyright law and just think it's cool to swear on the internet then the WTFPL will be fine.