Ok, as far as I can tell, the real story is that Engine Yard relies on a GFS/SAN setup that doesn't scale in the unique way that Github needs.

If you think about it, Github is one of the few sites that actually directly uses the filesystem heavily. Everyone else hits scaling issues on the DB first.

  The sad thing of all of this is it's not really a matter  
  of scaling, and it never has been. Our bottleneck has 
  always been the file system. GFS just... sucks. I'm sorry, 
  but I have to say it. Case in point, your graph. The first 
  rebuild I ran timed out because of GFS. The second one ran 
  fine, took maybe a minute to process, if that. GFS impacts 
  everything... gem build failures due to cloning... GFS. 
  Network graphs taking long time to build... GFS. Caching 
  jobs not completing... GFS. I think you see where I'm 
  going here. There's no plans to deploy the new code to the 
  live servers, and I think the reason is that we're afraid 
  it'll make GFS performance worse, not better. But on the 
  new servers where we don't have to fight GFS, it's 
  amazing.