Both Toon Zone and ICv2 have written of a supposed Entertainment Tonight report that there will be a sequel to the recently-released Transformers movie (which has already been a line of toys, several animated TV series, a comic book and a feature-length animated film).
However, my scan of the ET site has not brought up anything of the sort, and while a couple articles from other sites do allude to the possibility of a sequel, I see nothing that could be construed as an official announcement.
Personally, I think they will make a sequel, and director Michael Bay himself has said he had a lot of ideas he wanted to try that didn't fit into Transformers. Also, a sequel would open up the possibility of even more Transformers, including the Dinobots and Constructicons.
Also, star Shia LaBeouf has signed on to return if another movie gets made, which makes a sequel even more of a possibility. I doubt Hollywood is going to walk away from a chance to squeeze more money from those of us who grew up with the Transformers. Besides, I'd rather see another Transformers movie than a Thundercats one.