As a huge Magnus fan myself, I have to wonder if he's really necessary in the movie. While I would certainly love to see a "movie verse" toy of him in some form, I think if they worked him into the movie he would be relegated to a fairly small role and then Magnus fans would just feel cheated.
They've already developed the "core" team of Prime, 'Bee, Ironhide and Ratchet, and after adding the Twins, Sideswipe, and the Arcee sisters, that's a lot of bots to keep track of in a ~2 hour movie. I just think that Magnus, being a fairly high-ranking Autobot and not to mention huge, would require a considerable amount of screen-time to do him justice (and to justify creating the CG model) and I doubt he'll find it in TF3.
Plus I think Magnus might have once said some derogatory comments about Mr. Bay?

EDIT: BTW, I forgot to mention that that alt mode would make a great Magnus.
EDIT #2: Looking at the other "screen-times" thread kicking around, it seems that Bay is not against having intricate CG Models built for mere minutes of screen time...
