glibc is released under the LGPL, which specifically permits derivative works to make use of its functionality without "tainting" the derivative works' licensing. See: http://en.wikipedia.org/wiki/GNU_C_Library
GPL contains the viral licensing stipulation, and WordPress' use of the GPL license means that derivative works that are released are obligated to be licensed with a GPL-compatible license. Derivative works you make but don't release can be licensed however you want.
Excellent introduction! We know that the documentation is weak right now. The API is really starting to settle down, so now that it's less of a moving target documentation will get a greater priority.
And of course, you're invited to help us generate that documentation! Your introduction here demonstrates a terrific understanding of what new plugin authors need to gain proficiency.