I think there are two basic options.
Ask your superior WHY you are being assigned to this instead of what you were trained for. This might or might not provide you with the answer you want but knowing the motives will clear up a lot of unneeded questions. You might think management would explain themselves better without being asked but that's often not the case.
Just put your head down and do it.
There are several reasons for this:
a. You will be learning something new. All the back end skill in the world is useless unless the entire package is what the user wants to use. This is your chance to actually get close to your users and I know some programmers who would die for the opportunity. It might be a little painful at first but seriously, it's not often that the company is "paying" you to learn something on the job.
b. Understanding why front end guys do the things they do will make you a better back end coder in the long run. You will never be able to understand their needs unless you have really been in their shoes.
c. It's good for promotions and moving up the career ladder if that is what you want. As a tech lead and project manager, I will almost always pick the one that has seen both sides to take a supervisory role if one is available. So don't restrict your own options this early in the game.