http://download.oracle.com/otndocs/jcp/7224-javabeans-1.01-fr-spec-oth-JSpec/
http://www.oracle.com/technetwork/java/javase/tech/index-jsp-138795.html
Also, a direct link to the (PDF) specification.
Section 8.8 in the linked document is entitled “Capitalization of inferred names” and briefly outlines how names of properties are derived.