I uploaded a package designed to contain code useful for deploying to Heroku that I am using to deploy an app to Heroku that uses their free database [1]. All the package does now is parse the DATABASE_URL environment variable to help with connecting to the database. I will be doing a blog post on the Yesod blog shortly about deploying Yesod to Heroku, although everything is fairly well documented already [2].
Previously a loli app was deployed to Heroku by Mark Wotton (in addition to a Yesod app), and dbpatterson had (I am assuming unresolved) issues with deploying a Snap app.

[1] http://hackage.haskell.org/package/heroku-0.1
[2] https://github.com/snoyberg/yesod/blob/master/scaffold/deploy/Procfile.cg
[3] https://github.com/mwotton/heroku_haskell_demo