Today I learned that the so-called ‘Iron Triangle’ was invented by Dr. Martin Barnes. Therefore it should really be called the Barnes Triangle. There is an interview of him in the APM ‘magazine’ project  - the voice of project management - 11.2012 . I wonder why the namesake didn’t stick, as it has done with Maslow’s Hierarchy of Needs and Henry Gantt’s Gantt Chart.
The interview with Dr. Barnes was videoed and is currently (18th December 2012) available to watch (free) on the homepage of The PM Channel, as is the PMI Synergi 2011 presentation by yours truly on Project Management in SchoolsProject Management Learning in Schools (which you would have to pay to access, but is free to PMI UK Chapter members).
The problem being that there are lots of variations in the names for each element of the iron triangle, and the centre of the triangle (usually seen as time, cost, and scope with quality in the centre). The Barnes triangle has the elements time, cost, and performance, with risk in the centre. I prefer the Barnes triangle, especially as a visual metaphor where one element or ‘side’ of the triangle erodes or breaks through mismanagement and risks escape.