Can my employer drastically change my job description? If I'm hired for sales can my company - at will - make me a janitor? Or, in this case, customer service? What if I'm ill-suited to my 'new' job? Do I have any legal recourse? Thanks!

Depends on the fine print of your job description. Some will state that you will perform these duties and "any other that the company sees fit." That's where they get you. If they do move you to another job that you are not qualified for and give you a negative evaluation you can state in writing on that evaluation that you have not been trained for the position nor was that what you were hired for. That does not mean they have to place you in your former position, only that if they fire you for poor performance you can draw unemployment compensation or you might seek to sue for wrongful dismissal. As the previous respondent asked, depends on your state.