The old saying is specific to Black Locust, as for as I know, and an exagge
rated 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 tree
s, hence, lasts a long time.
Our old homestead had a fence line, as this, until Hurricane Gustav blew do
wn most of the trees. I had some of the upper trunks milled and the sawdus
t or unseen chemistry, when working the subsequent boards, was very irritat
ing, 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 be
nch from that catalpa cache - http://www.flickr.com/photos/43836144@N04/532