The issue with heroku and/or GAE is that they are single services.  They have and will continue to have unplanned downtime.  The issue here is mitigating a single service's downtime by having redundancy.  Redundancy almost always involves duplication of resources.  Essentially, you could switch to GAE or Heroku, but you haven't gained much, and possibly lost, since GAE apps can usually ONLY run on GAE.