Ok, lets see. The languages that I have seen come and go as intense fads:
* Easel
* Telon
* Powerbuilder
* Codfusion
* CICS (was actually taught in a "CS" course not too far from here)
* PL/I (may be not totally dead, sadly)
* IITRAN (maybe not that intense)
To Tom's point, I know of a project involving 300 Telon programmers who had to be let go when a massive project failed. Few who invested in the above technologies are working in them today.
More importantly than the article says, develop skill in solving business problems using your programming skills.
None of these are programming languages in the sense people normally use the term (with the exception of PL/1). They are propriety frameworks and tools. Even PL/1 was an IBM developed language.
Proprietary frameworks, products, and tools are definitely a terrible thing to base your career on. Languages such as C, C++, Java, Ruby, etc. are a good investment.
I was maintaining a custom piece of PL/I software until 2010 when I changed jobs, as far as I know the place is still using it. I suppose I have potential value as one of the few people (barely) under the age of 40 that have experience in it.
More importantly than the article says, develop skill in solving business problems using your programming skills.