At present there are no details about her character. As for Huntington-Whiteley, aside from Victoria Secret, she is also Jason Statham's girlfriend.
As far as her acting resume, this is non-existent (except for playing herself on Victoria Secret Fashion Shows). However, Megan Fox was an unknown when Michael Bay cast her in the first Transformers film so there could actually be something to this story.
It seems that with all the money that it will cost to bring this film to the big screen, Fox's replacement will be announced by DreamWorks/Paramount sooner rather than later.
Transformers: Dark of the Moon was released June 29th, 2011 and stars Shia LaBeouf, Rosie Huntington-Whiteley, Julie White, Kevin Dunn, Josh Duhamel, Tyrese Gibson, Alan Tudyk, John Malkovich. The film is directed by Michael Bay.