Wikipedia wasn't very forthcoming on what happens to children born overseas to US-citizen parents (such a child born in the UK would not get UK citizenship), but they did at least link to the actual law, which says that a child both of whose parents are US citizens is a "national and citizen at birth" whether born in the US, in an "outlying possession", or anywhere else. This surely includes McCain.

(I initially read "McCain's Canal Zone Birthplace Raises Eligibility Questions" as "McCain's Birth Canal Zone Raises Eligibility Questions", which would certainly have raised questions although perhaps not about eligibility.)

Peter