It appears that the planet-eating monstosity known as Unicron (Above in Planet mode) may feature in Transformers 3. Although their have been rumors on many websites about a large unknown robot being featured in the final scene`s, This is the first time it has been mentioned in a post as credible info.
"Supposedly, we also will be getting a look at Unicron which if done right, will be phenomenal"
Also Transformers 3 has been listed as number 10 in the top 10 most anticipated 2011 sequels on this website. Suprisingly this mentions preview footage of Unicron (Possibly from the ROTF extra`s)
"10. Transformers 3 A darker more introspective journey into the Transformers universe and their origins on Cybertron. There is also preview footage of the possibility of Unicron, the planet eater, in this sequel. In Movie Theaters:July 1, 2011"