Think carefully. If you are not successful with your job search (and that has already happened to you once, apparently, as you did not find the job around the time you defended), and you decline to run the seminar, you will spoil your relations with the department chair/head.
To other previous responders: come on, guys, don't you know who's who in the academic food chain positions? The leader of the group considers him/herself busier than other members of the group. Other professors are busy with the research, their own grad students, and other committee assignments, and don't like to be bothered by additional service (which they also view as the lowest priority job, as clearly, and to the point, communicated by Thomas. So the group leader went to the person with the least negotiating power and the greatest amount of time, at least as perceived by other professors -- the new Ph.D., arguably with no other service assignments, no students to teach, etc.
So the overall advice is to weigh your options. If you are certain that you'll find a job elsewhere, feel free to decline. Keep in mind that this is an intertwined issue in that your group leader is to write your recommendation letters, and if you are perceived as non-cooperative colleague, that will make a way to the letters and (fewer) interview invitations. I was given the department seminar series to run in my first year out of Ph.D. tenure track job, considered it fun, but got kicked out of my tenure-track job two years later as I was perceived as doing too much service, and too little research. If you will have the opportunity for a greater deal of interaction with the invited speakers than just sending email on your leader's behalf, this could be valuable. If all you will find yourself doing is reservations and such, then this may not be as good. However, if you have a regular date and time for your meetings, you should be able to reserve the room once for the whole semester.