My specific case involves the Qt SDK which I am using under the Lesser General Public License. It states that I must release any modifications to the licensed software package that I make.
Intellectual Property Law Attorney
Very simple answer: most likely. I suppose if the class was somehow public domain or freeware, maybe you would not have a problem. But if the class is a copyrighted part of Qt, you must abide by the license. A derivative work is most likely deemed a modification, and your subclass is a derivative work. License compliance, particularly, with open source licenses can be very complex. You should consult an attorney. If you cannot find someone nearby you in the San Diego area, please feel free to contact my office. I handle software IP matters, with a particular emphasis in open source.