If you happen to see this error after upgrading your SliceHost account, be calm.

ActionView::TemplateError (Define INLINEDIR or HOME in your environment and try again)

The simple solution is to add an environment declaration to your environment/production.rb file:

ENV['INLINEDIR'] = '/path/to/.ruby_inline'

Once that was set, we were in business again… whew!

References

Here's a relevant reference that took a while to find: