Effect of "D" grade in PhD-level course on job prospects
Its not the grade that you get that determines gets you the job.  It's your ability to apply what you have learned to new and challenging fields that ultimately determines your qualification for a job.  This is best measured not by grades, but by the quality (and possibly quantity) of publications.   In rare circumstances, as @aeismail suggests, employers look closely at your GPA.  I do not believe this is the norm though.  Most institutions that employ the highly educated look more highly upon strong letters of recommendation and publication history.  As long as you have that, you should not have any problems with this grade on your record.

The ultimate impact of your "D" really depends on the policy of your school.  As @BenNorris indicates, some graduate programs have a minimum performance requirement such that a low grade in a class may impact your retention in your program.  It is best to speak with your advisor, the graduate chair, and possibly even your department chair to discuss the circumstances surrounding your performance, how it will impact your retention, and if there is anything you can do about it.  Immediate dismissal is extremely rare.  Most institutions place the student on probation (and may make you repeat the course in question).