* If JRuby got all the nice operational support that Heroku currently gives CRuby, it would be an awesome way to add power to an existing Ruby codebase (run the same code in JRuby, give or take a few gems, start taking advantage of Java libraries or using threads for better throughput).
* Heroku supporting Scala and Clojure could really boost their popularity - AFAIK there's no service for those languages that makes deploying a production-ready, public-facing web app as easy as Heroku does for Ruby.
Update - I guess this quote from the article is relevant, and could indicate a reluctance to support other JVM languages:
Though growing quickly, the Clojure community is small enough to be approachable and accepting of new ideas. This is crucial for a platform like Heroku, which offers a deployment workflow that is a radical departure from that used for server-based deployments. Language communities with heavy investment in traditional deployment methods will be harder to adapt to the Heroku way.