With Lisp, the killer app is the language itself.
I would emphasize that the killer app for Lisp is simply programming. "Simply" in the sense that you're unencumbered and not fenced in to any particular cliche such as Ruby being stereotyped in its association with Rails.
Ten years ago when deep into Python, I'd explain the language to others by including the disclaimer that you probably wouldn't want to use it for writing an OS. Writing a near-deterministic scheduler in it ultimately motivated my return to Lisp.
I offer no such disclaimer when describing Common Lisp to other programmers, executives or founders at start-ups where I've used it.
Apparently, most people want to convince themselves of the value for a tool by citing something external. To this end, it's likely that the success of EVE Online has helped Stackless/Python in terms of reputation.
I suspect everyone paid to write Lisp that I know would respond with something like: if you need that kind of external validation, then you probably don't know what you really need or want, so let's not wake the sleepwalker.
Maybe the sociological question has to do with why some people need/want that validation (killer app) versus those who don't.
For completeness and to preempt questions of what is the EVE equivalent for Lisp success stories: early days of NaughtyDog using GOAL, ongoing work at the University of Colorado and 2010 book Land of Lisp teaching Lisp "one game at a time", just to name a few.
But that will never be enough for people who prefer commodity languages for the sake of safety in numbers, as Lisp isn't commodity today. More than just a bootstrapping issue or citing decline due to the AI winter, it may never reach critical mass to be commodity because so few individuals are involved with each success story.
That is, since you often need only a few developers when using Lisp, there's less need to evangelize when recruiting. Job postings I've made have often been a single sentence mentioning Common Lisp and the city, which yielded very high quality people. Contrast that with Zillow.com's sly marketing which encouraged its hundred employees at the time (mostly Java devs) to vote for best employer, which helped them double by end of 2007.
It's rather like what is said of big dogs: they don't bark nearly as much as little ones because there's nothing to prove.
For Lisp, questions of killer apps and becoming a mainstream or commodity language (again) is unfortunately impacted by this larger context. Certainly, I would love to see my favorite language enjoying popular status. In the end, however, it's rather like some explanations for the meaning of life: it's a personal quest, and the journey is part of the answer. So too with deciding to use Lisp. It's okay to break-away from the crowd.
Try it and decide for yourself. Or Don't.