The notion that the Book of Mormon peoples were not alone in the Promised Land has been around for decades. LDS Scholars today are basically unanimous in the belief that they (the Nephites and the Lamanites) met and interacted, and even intermixed with other groups and civilizations. However, it must be admitted that most of the evidence drawn out of the Book of Mormon for such a belief is indirect. There seems to be no passage which explicitly mentions an encounter with another group of people (besides the Mulekites, who were from the Old World like the Nephites).
Recently, I came across a passage, which I have yet to see cited by those who support this view, that I believe it the most explicit (not to mention the earliest) reference to other people that the Lehites may have encountered.