Although the official line up has not been posted, the names of most performers for this years Coachella Music and Arts Festival (a 3-day music festival in Indio, CA) have been released. Amongst these rumored artists stands Britney Spears. I was (quietly) informed by a friend at Goldenvoice that Britney is a 100% confirmed headliner. The dates for this years Coachella are April 17th-19th, allowing Britney to do her Circus tour as planned. Her Southern California tour dates are set during the same week as Coachella. Britney does not have any booked dates on Coachella weekend, in order to headline either saturday or sunday at the 65,000 guest festival.
Source: good friend/insider at Goldenvoice
EDIT: brit brit isn't headlining but she will be performing. probably a short set in a tent.