There's a Metro stop (Ottaviano - Line A toward Battistini) not too far from St. Peter's - I walked it today. Lots of people were walking there from this stop. After two weeks in Italy my feet are almost on strike, but I made it just fine - not very far. There's also the #40 express bus from Termini to even closer to St. Peters (that's the last stop) - took that today also. I don't like long bus rides and my bus was full of noisy school kids, and even the express seemed to take forever, but it may have been the most convenient, least confusing way to get between Termini and St. Peter's. Exit Termini, find the array of buses, ask where the #40 is, it begins at Termini and ends at St. Peter's.