This is because Paramount is now opting for the simpler title "Transformers: Revenge of the Fallen". In related news, Jalopnik has posted spy photos of the new Bumblebee (the 2010 Chevy Camaro Z28) as well as an unknown Corvette concept.
Please note that, in order to be concise, the film's title will still be referred to (by fans and by site staff) as either "TF2" or "ROTF." It is also worth noting that Michael Bay's official site still has the title listed as "Transformers 2: Revenge of the Fallen".
Strangely.
Source: Superherohype
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Fan sites was informed that Transformers 2: Revenge of The Fallen will no longer have '2' on it. While it was previously "announced" as "Transformers 2: Revenge of the Fallen," Superhero Hype! has learned that DreamWorks/Paramount has opted to make it Transformers: Revenge of the Fallen instead.