Britney to Appear During Madonna Concert

Spears had been scheduled to begin shooting a video montage for Madonna's upcoming tour this morning at the Lots soundstage in Hollywood. But production insiders say that because the set was not finished in time, cameras will not roll until tomorrow.

The set features an elevator, in, around and on which Spears will dance. "The video will be very deep," a source in the Spears camp tells E! News. "You've never seen Britney like this before. It will blow your mind."

Madonna reached out to Britney to do the project, says the source. The Material One's publicist, Liz Rosenberg, would only say that "there is footage being shot of Britney some time this week in conjunction with Madonna's upcoming tour" and that the segment was not a music video. "That is all the information I have available," the rep added. "The rest is a secret."