The BBC announced this morning that Karen is set to star in a BBC4 production called "We'll Take Manhattan", about 60s model Jean Shrimpton and her affair with photographer David Bailey.
The part of Bailey has yet to be cast. Filming is set to start in May; a transmission date has yet to be announced, but will probably be later in the year.
Source.
What with this and Romeo & Brittney also supposedly starting to film in May--could be loose confirmation that Karen and Arthur are indeed leaving after Series 6?