I earned my Ph.D. in Materials Science in 1992, and my PE license in Metallurgy in January of this year. I have worked for the same employer as a Senior Process Metallurgist in the specialty steel industry for the past ten years, and have concurrently worked as an adjunct Associate Professor in the College of Engineering at Syracuse University for the past 9 years. Jakin is correct in stating that most university professors in United States that work in engineering colleges have not earned a PE license. The vast majority of them are not required to do so. As far as I know, I am one of only two professors in Engineering at SU that is licensed. In answer to hydrae's question, writing technical papers for publication in scientific research journals does not require a PE stamp. However, writing such papers will provide the practicing PE with continuing education credits, which are required in certain states.
Several of my former professors in graduate school were PEs. At my alma mater they HAD to earn their PE licenses to keep their jobs. Back in the 1980s ABET noticed that a significant percentage of the tenured faculty members in my engineering school did not in fact have engineering degrees. Many of them had degrees in related fields such as physics, chemistry, applied mathematics, etc. They argued that these professors were not in fact engineers, and if this college was to maintain its current ABET status, then they would have to hire faculty that had engineering backgrounds or risk losing their accredidation. As a result, the professors all eventually took the PE exams. By passing this exam, it gave them legal recognition as practicing engineers. As a result, the college maintained its ABET status.
The complaint that I most often hear from students regarding faculty members is that their other professors don't have any "real world" experience. This is, in most cases, an accurate statement. Most of the other faculty members that I work with have never held a job in an industrial environment. And they can't teach their students what they don't know themselves. So students looking for actual examples of what they will encounter in the job market when they graduate are usually sorely disappointed. And these students are, in my opinion, short-changed in terms of their education. The reason that most engineering colleges do not require their faculty members to become licensed is that they are not required to do so. And these programs don't view themselves as producing engineers anyway. They are producing the next generation of researchers and scientists. Exceptions to this include engineering schools such as RIT, where practical engineering courses are a priority.
Beware of the expert claiming that you should judge his expertise and ability solely on the degrees that hang on his wall. The PE license tells you that he is "minimally competent". The Ph.D. tells you that he has a great deal of perserverence, and deep knowledge in a relatively narrow field. These degrees tell you nothing of his character, competence, or his integrity.
Maui