Hi co-founder of SpacialDB here. Cloud hosted PostGIS is already available as a Heroku plugin, stand-alone and an easy to use API. Check out http://devcenter.spacialdb.com/ for documentation and you can sign up via the Heroku addons page or http://beta.spacialdb.com/

PostGIS is currently in a pilot phase. If you're serious, please contact me as described elsewhere on this thread.