I have often heard that gold and salt were sometimes considered of equal value. When and where did that occur?
During the era when the Phoenicians ruled the Mediterranean sea and surrounding territories (cerca 1550 - 300 B.C.), salt was indeed a highly precious commodity. After this, the Romans became the dominant force in the Mediterranean, though the value of salt did not immediately decline by any means. Whether it was pound-for-pound as valuable as gold, I think it is hard to say, but probably not out of the question at some point.
Until relatively modern times, salt was prized mainly for its ability to preserve foodstuffs as well as season food. In the hot Mediterranean climate where fish and meat would decay rather quickly, this was particularly useful. The period of Phonetician domination during the early to mid first millennium B.C. (including its successor state Carthage) represented one of the world's first monopolies. Essentially, other nations (including Ancient Greece and early Rome) did not have great access to salt, and hence the Phoenecian monopoly could dictate its own outrageous rates.
From this article on the economy of the Phoenicians:
I find it fascinating that the modern English word "salary" (pay to a worker) derives the Latin "salarium", itself deriving from the Latin "salis" for salt. Whether Roman soldiers were actually paid in salt sometimes (quite debatable), they often would have bought salt with their pay, given its utility and valuable nature. Either way, this notable etymology, along with a number of common modern quotes in English, alone provide good support for the precious role salt once played in civilisation.
I don't think that gold and salt ever were equal in value, that's an exaggeration. Salt was very valuable however, particularly because of its use for conservation - valuable enough to make one very rich. This allowed cities that sold salt (e.g. Lüneburg) to get very wealthy and influential. The Wieliczka salt mine supposedly was responsible for one third of the Polish state income between 14th and 16th century. But they obviously had to sell salt in large quantities for that.