There are different types of Master's programs- some require everyone to get one and some say it's "optional." The ones I can think of off-hand: UNC (3 years, Masters required), Baylor (option), UCSF (option), UAB (option), USC (option), U Conn (option), Iowa (option), Indiana (option). If you get your Master's it is usually at least 4 moths to a year extra. The AAPD website has a list of all the programs and what they offer.
http://www.aapd.org/training/
I graduated from Baylor in 2003. When I was there most of the candidates we interviewed had Part I scores in the low 90's and were in the top 1/3 of their class. We even had a #1 with 99 on Part I. That being sad, Baylor is a very competitive program. Pedo has become a lot more competitive than when I applied way back in 2001! Not all programs want you to have 90+, but It would help.
I had several friends in dental school and students I taught in residency who didn't get interviews because they weren't competitive enough. Also a lot who got one or two interviews but didn't match. I am not trying to make it out too be as competitive as Ortho or OMFS but the days of pedo being the "easy" residency to get in are long gone. Just make sure you do well in school and study for Part I. And yes, the interview is a big deal. We had many people say stupid things and get dropped to the very bottom of the list or not ranked at all. So yeah, the interview is important but they don't interview everyone who applies. My last year at Baylor we had 100 applicants and we interview 25-30 for 8 spots