This is in no way an official confirmation or news. Movie website, CHUD, has posted up a tidbit from an e-mail they received:
“Evening Nick, my mate is working on Prometheus as OMITTED and met Guy Pierce on set today. She’s previously worked on OMITTED, OMITTED and OMITTED and I’ve no reason to believe she’s lying. I’ve not read anywhere about Guy being cast so thought it might interest you.“
They posted an additional comment to say since posting the article they’ve received 2 more e-mails confirming the initial post. I’ve also asked Fox for comments. Watch this space. Thanks to Darkoo for the link.
UPDATE: It has now been confirmed by PlayList that Guy Pearce has in fact joined the cast of Prometheus.