It's a wild rumour at the moment, based entirely on the fact that Charlize Theron is under contract with 20th Century Fox for at least one more movie and has recenlt dropped out of Clint Eastwood's J. Edgar Hoover biopic, but there's the chance that Miss Theron may well land a part in the new Alien prequel movie directed by Ridley Scott.
Scott wants the lead part in the film to be played by Swedish actress Noomi Rapace (Sherlock Holmes 2) who would play the lead role of Elizabeth Shaw. Charlize, if she is chosen, would be up for the part of 'Vickers' a fortysomething, tough-but-sexy crew member which Scott originally wanted to give to Michelle Yeoh (Tomorrow Never Dies).
Noomi Rapace
Like I said though, it's just a a rumour and with no proof or confirmation just yet.
UPDATE: Immediately after posting this I've read that Charlize may now have chosen to take the part of the Evil Queen in the new "Snow White and the Huntsman" movie... Maybe no Alien movie after all!