Is there a good, unbiased, secular retelling of the life of the Muslim prophet, Muhammad?

There have certainly been attempts. The problem is that the sources that we have for Muhammed's life, other than the Quran (which as a non-narrative text isn't very good for biography) and the constitution of Medina, were both religious and written a couple hundred years after Muhammad's death.

So the real division isn't between secular/religious, it's between biographies of the prophet that rely on the traditional sources (i.e. effectively all of them) or those that don't (i.e. in which case we know virtually nothing about the man.)

There's a fantastically succinct essay explaining this problem in greater detail by Robert Hoyland entitled Writing the Biography of the Prophet Muhammad: Problems and Solutions which you can see here (PDF warning).

edit: I think I should perhaps expand a bit in case anyone hasn't read these kind of texts, or doesn't want to read Hoyland's essay. The basic problem is that there's a lot of "Jefferson bible" sort of biographies of the prophet, where they take these handed down traditions (i.e. the hadith, the sira, the tafsir), ignore the religious bits, and report the rest as fact. The problem is that source skeptical historians argue that all of those sources are unreliable, not because they include religious bits!!!!, but because they were handed down in such a way as to be fundamentally unreliable such that we have almost constantly contradictory information about basic questions about Muhammad's life. There are still academic historians who will defend these sources, and I myself am not a complete traditional-source-skeptic, but there is no consensus at all, academically, about whether these sources can be used or are totally bunk. As a result, from a western academic perspective, we don't really have any fully reliable retellings of Muhammed's life at all-- religious or otherwise.