At SpringOne 2GX I did a presentation about running Spring Roo (really Spring anything) and Grails apps on the cloud with Heroku. A video recording of my session has been posted on InfoQ.
If you want to try out Spring Roo or Grails on Heroku then one way is to deploy a copy of one of the demos I showed. I’ve created a little app that makes that easy. Just go to java.herokuapp.com and select a demo then enter your email address (your Heroku username) and click Go! A copy of the demo will be deployed for you on Heroku and then you will see instructions on how to get started with it. Give it a try and let me know what you think.