I read that she did admit to using a body double.
This was on one of our local radio station's site:
Teenage singer/actress MILEY CYRUS has confessed to using a body double during concert performances so she has time to make costume changes.
The star was caught out by eagle-eyed fans who spotted a lookalike onstage during a recent show on the sold-out Miley Cyrus/Hannah Montana Best Of Both Worlds tour.
But the 15-year-old's representative insists a double is a necessity of the performance, telling OK! magazine, "To help speed the transition from Hannah to Miley, there is a production element during the performance of "We Got The Party" incorporating a body double for Miley.
"After Hannah has completed the featured verse on the duet with the Jonas Brothers, a body double appears approximately one to two minutes prior to the end of the song in order to allow Miley to remove the Hannah wig and costume and transform into Miley for her solo set.
"Other than during this very brief transitional moment in the show, Miley performs live during the entirety of both the Hannah and Miley segments of the concert."
Copyright World Entertainment News Network 2008
To me, I didn't make much sense to use a body double for appx 1 - 2 minutes. When we saw her in Cincinnati, she sang with the Jonas Brothers, then they stayed on stage while she "changed into" Miley Cyrus. They even said we are going to sing a couple songs while Hannah Montana changes into Miley Cyrus. Then they went off stage, we waited a short bit then Miley Cyrus came out. Since the Jonas Brothers sang while she was changing and we waited a few moments anyway, why use a body double for "appx 1-2 minutes"? I don't like the fact that she used one at all.

I'm curious if anyone thinks a body double was used for more than "appx 1 -2 minutes".