To answer your question, there are currently nine dioceses in the United States that are without a bishop. I know some have been vacant longer than others. My opinion on this is that the Papal Nuncio for the United States and Pope Francis I are striving to find the “right bishop” for the job.
Some dioceses have become vacant because the previous bishop was elevated to Archbishop (this is the case of three current vacancies, four if you include that another bishop became a coadjutor), and some bishops have gone from smaller sees to larger ones. One diocese is vacant due to a resignation for health reasons, and another is vacant (sadly) because the bishop died suddenly.
I also think some dioceses are more urgent for replacements. For example, I am glad Rochester (NY) was filled before Portland (Maine) and Fort Worth. Currently, Portland (Maine) has the longest vacancy.
I’ve heard a one year wait for a bishop is common, but I sure would like to see Albany (NY) get a good, new bishop soon. The current bishop (Hubbard) submitted his papers to Rome days after his 75th birthday, and I hope and pray that Hubbard’s retirement will be accepted quickly (Bishop Clark’s from Rochester were approved two weeks after his 75th birthday).
I will make a short post on which dioceses are currently vacant as of November 23, 2013. I got this list from Ed Peter’s website, which is canonlaw.info.
I hope that helps a little.