The question in trademark cases is, essentially, could a reasonable person be confused as to the source of a product or service bearing a trademarked term, image, design, etc.? The idea is that we don't want someone to be able to, say, sell a brown-colored (or potentially any other colored, nowadays) beverage and have a reasonable person think it comes from Pepsi. Or, trick someone into buying a cheap knockoff shoe by making them think it's made by Nike.
In this case, then, could someone think that the Transformer Prime is made by Hasbro? Are tablets like the Prime close enough to Transformer toys that a reasonable person is going to be confused as to their source?
I'm guessing, probably not, because Hasbro didn't sue ASUS over the name "Transformer," even though in fact the tablet "transforms" from one kind of device (a tablet) into another (a notebook computer) in much the same way that Transformers transform from robotic creatures into recognizable human vehicles. Either Hasbro didn't see a case, or there was a quiet license agreement cut between ASUS and Hasbro.
Interestingly, "Transformer Prime" takes the use even further away from the generic concept of "transforming from one thing to another" to a specific "Transformer" character, i.e., Optimus Prime. The word "Prime" in this case is very intimately tied to the Hasbro franchise, creatively. It's a bit of a stretch for ASUS to say "we really just mean that this tablet is the 'prime' example of a transforming device." I mean, seriously.
Again, though, tablets aren't the same as toys, and so although ASUS benefits from the connection made by fans of the Transformer cartoons and movies and such, there's not likely to be any confusion among consumers as to who is making what. Hence, there's probably no trademark infringement case.