According to Rick Steves "updates" to his 2009 guide, after it was published:
"St. Peter's Basilica now closes at 17:00 Mon–Fri, and at 16:00 on Sundays and in the winter (Oct–March)."
Is this accurate, though? The official Vatican website still hasn't been updated to reflect these changes, if they are true. Looks like the closing time is still 18:30 (Oct-Mar) or 19:00 (Apr-Sept).
http://www.vatican.va/various/basiliche/san_pietro/it/basilica/orari.htm