The old saying is specific to Black Locust, as for as I know, and an exaggerated compliment to its longevity as fence post material.... and, in effect, seconding Karl's opinion/fact of BL being the best wood for posts.

Additionally, getting away from helping the OP:
In some cases, catalpa makes for long lasting fence posts, despite the wood being soft and porous. Many split rail posts take root and grow into trees, hence, lasts a long time.

Our old homestead had a fence line, as this, until Hurricane Gustav blew down most of the trees. I had some of the upper trunks milled and the sawdust or unseen chemistry, when working the subsequent boards, was very irritating, even when slightly inhaled. That was some potent stuff, when sawing, working with it. The wood is a pretty, chocolate brown. Here's a small bench from that catalpa cache - http://www.flickr.com/photos/4383614...in/photostream

Sonny