It's fine to call central America that if you wish, in the same way I can call Japan+Korea+Thailand etc East Asia, but that doesn't mean East Asia is it's own continent... you can generalise areas however you want, but in terms of what countries are in which continent, officially all of what you call central America is part of the North American continent.
I think you could argue that geographically the Panama canal is the line between N.A and S.A, but politically all of Panama is in N.A.
There are other geographical splits that I don't think line up exactly with political boundries, for instance part of Egypt is really in Asia.
I think there are only two countries that are split between continent politically though, those being Russia and Kazakstan, at least according to a map i used to have on my wall (up to date for 2003) both those countries seem to be part European and part Asian.
