Circus is this year's show from Britney Spears. It started out in the US in the spring then made the leap over the Atlantic to Europe for a couple of months. After draining the pockets of prepubescent tots everywhere it will return to the US for a second run in August.
There have been reports that Britney Spears isn't so much singing her way through the tour as in fact dancing her way through it: the singing was alldone some months ago and kept on tape. However, given her background as a dancer rather than a singer, this probably isn't all that much of a surprise.
It's also most unlikely to be a surprise to her fans: we're not really talking about people lining up to hear a world class voice here, rather, to see the whole spectacle.