A nascent list of primarily UK 'celebrities' currently using twitter (and a list of confirmed fakes):

Comedians:

Russell Brand

Rob Brydon

Alan Carr

John Cleese

Stephen Fry

Russell Howard

David Mitchell

Dylan Moran

Jonathan Ross

Literary:

Neil Gaiman 

Miscellaneous:

Fearne Cotton

George Lucas

Chris Moyles

Jamie Oliver

Andi Peters

Musical:

Roots Manuva

Sam Sparro

Political:

Boris Johnson

Barack Obama

Arnold Schwarzenegger

(See also: Tweetminster for a full list)

Sporting:

Lance Armstrong

Will Carling

Andy Murray

See also this list, including some of the above from The Observer and another from The Times.

Thanks

Pete Goold - Managing Director of Digital PR Agency Punch Communications - PR Agency Homepage / Punch Blog / Twitter / Linked In

Punch team Twitter details: Abbie, Ben, Claire, Christie, Katharine, Pete, Philip