The term technical debt was coined by Ward Cunningham to describe the obligation that a software organization incurs when it chooses a design or construction approach that's expedient in the short term but that increases complexity and is more costly in the long term. Ward didn't develop the metaphor in very much depth. The few other people who have discussed technical debt seem to use the metaphor mainly to communicate the concept to technical staff. I agree that it's a useful metaphor for communicating with technical staff, but I'm more interested in the metaphor's incredibly rich ability to explain a critical technical concept to non-technical project stakeholders.
Update Feb 20 2012: the webcast replay links above no longer work, but Construx has posted the webcast on YouTube.
And they posted the slides on SlideShare: