The Sun claims that Madonna has asked Britney Spears to join her on tour for a few dates, in a move to help Spears get her actual career (singing, etc, as opposed to being paparazzi/tabloid fodder) moving a bit.
Apparently, Britney is to appear on a video recording due to be screened during Madonna's upcoming Sticky And Sweet dates, but may also put in some live appearances. The Sun's source is quoted as saying: "They are recording a video together on Sunday that will provide the backdrop for part of Madonna's show. And they are talking about Britney taking the stage on a few of the dates too. It's hoped that Madonna can help Britney get back to her old self".