Rosie Huntington-Whiteley's involvement in Transformers 3 was close to official thanks to reports suggesting that an offer was out for the model-turning-actress to accept or reject. The casting became a bit more official after Victoria's Secret released a congratulatory video on Huntington-Whiteley's behalf regarding the sci-fi project.
But if it wasn't officially official before, it is now: Michael Bay's own official site is officially making Huntington-Whiteley's casting officially known:
I would like for all you S4TE fans to welcome [Rosie Huntington-Whiteley] to the world of Transformers. As some of you might know, she's the new female lead in Transformers 3. She's worked with Michael before in his Victoria's Secret "A Thousand Fantasies" ad. You can view it here and here.
So, there. Officially official. I'll be in the corner wishfully thinking that Huntington-Whiteley's character transforms into a monstrous metallic succubus and devours Sam Witwicky at the film's conclusion.
Source: Michael Bay