If there was a cover up, it wasn't very successful.
“I was under the impression that this was already well known.”
By non-Mormons, yes.
“If there was a cover up, it wasn’t very successful.”
Well, it was until recently. Many Mormons openly denied that Smith had multiple wives or if they admitted that much they said he did not consummate the marriages, the wives were old, infirm women who needed a man to support them, etc.
Then you have this bizarre thing: “Smith’s son Joseph Smith III and widow Emma Smith repeatedly taught that Smith opposed the practice and was not a polygamist.”
Smith III and Emma were the force behind organizing the RLDS (Reorganized Latter Day Saints), that is, the Mormons who stayed in Missouri and abandoned polygamy when everybody else went to Salt Lake. Today the organization is called the Community of Christ. The point is that Smith’s own son and his first wife vehemently denied that Smith practiced polygamy! Clearly they would know if he did or not, but the evidence always has been overwhelming that he did so they were lying through their teeth about it. http://en.wikipedia.org/wiki/Comparison_of_Community_of_Christ_and_The_Church_of_Jesus_Christ_of_Latter-day_Saints
I have encountered Mormons who knew nothing about Smith’s polygamy and other Mormons who knew about it but said the multiple marriages were of a spiritual and non-physical kind. How those two views co-exist among otherwise “orthodox” and “practicing” Mormons with Temple recommends is beyond me.