EWTN Catholic Q&A
When is Cardinal Ratziner the Pope?
Question from George on 04-20-2005:

is it when he accepted his election, or when inaugurated?

Answer by Colin B. Donovan, STL on 04-25-2005:

Since Joseph Ratzinger was already a bishop at the time of his election, the necessary pre-condition for being Bishop of Rome, he was Pope the instant he consented to election. Had he been a simple priest, following his consent he would have been immediately ordained a bishop, and in THAT moment he would have become Pope. So, the formula is "election + consent + episcopal order = pope". The Inaugural Mass was simply a formal, public, liturgical celebration of the beginning of his pontificate, which in fact began Tuesday evening, the 19th of April, 2005, when he accepted his election.