The mono build pack from Ben Hall (https://github.com/BenHall/heroku-buildpack-mono) looks interesting.
With AppHarbor's pricing announcements yesterday (which sucked) I've finally got around to looking at Mono and Heroku. It looks a good combination, although still referred to as unsupported/unstable. I plan to deploy a few test apps to Heroku using the mono build pack and NancyFX.