if using the API makes it a derivative work, then glibc is a derivative work of libc because they both implement the same API
all iphone apps are derivative works of the iphone OS as it created the API (This is something that Apple would love to have be the case, that way they could use copyright and the DMCA to ban anything from running on the iphone without their approval)
what's even worse is trying to say that using the API in one way (static linking) makes a derivative work, but in another way (dynamic linking) does not. Given that there is no difference in the creative work involved (writing the new program), only in the mechanical step of creating the binary.
The example of apple above is why I say that winning the definition war would be dangerous. It could get even worse than that example. Microsoft has now licensed the ARM core, so they may an ARM device that has a custom chip (in the ARM world you have the core and a bunch of things that can be combined together and manufactured as one chip), and then set the license for the chip API to exclude GPL code. They could even do it in an 'accidental' fashion like the Sun CDDL does.
some free software advocates are so eager to get legal tools in their hands to beat on proprietary software vendors that they don't stop to think how those same tools could be used against free software.