Since no one famous has been sued for copying the interface definition, the answer is trivially, "No". SCO keeps suing companies for actually copying their intellectual property. And they keep losing because they can't prove they actually bought it.
You can't copyright the interface idea, only the publication of the interface.
Copyright applies to a specific document or a specific pile of source code (which is considered a document). Go to the US copyright page for more information. It's pretty clear what you're protecting when you send stuff in for a copyright certificate.
http://www.copyright.gov/
An interface or a design might be patentable (it shouldn't be, but that's another story). But an interface cannot be copyrighted.
The interface documentation, the source code, any document can be copyrighted. But not an idea.
"But it's the exact same line of code". Right. But it's not the whole document. Copyright does not prevent all use. It gives you recourse when you find that someone has copied your document. Retyping a few lines of code here and there out of a larger work is protected fair use.
When you copyright source code, BTW, you don't send all the source to the Library of Congress, just 50 pages of source (first 25 and last 25). Enough to establish a claim on a the specific copy of the source.
A clean-room development (from scratch, without viewing the original source) circumvents copyright protection. It's a protected, widely-used practice. One person writes a specification, the other person writes the code. If -- coincidentally -- the code is similar, the "clean-room" means you guarantee that the person writing the code never saw the original, and it's coincidence.
Could someone possibly be sued? In the US, someone can always be sued.
From http://www.copyright.gov/circs/circ61.pdf
Copyright protection extends to all
the copyrightable expression embodied
in the computer program. Copyright
protection is not available for ideas,
program logic, algorithms, systems,
methods, concepts, or layouts.