In a chef run, when reloading ohai, e.g. ohai "reload" (https://github.com/flaccid/cookbooks/blob/master/cookbooks/ohai/recipes/reload_all_plugins.rb), node['cloud']['provider'] becomes unset and returns NoMethodError: undefined method `' for nil:NilClass.
Running chef again shows the node attribute is set again (i.e. its not persistently unset).
This appears to be an issue/bug with http://wiki.opscode.com/display/chef/Automatic+Attributes and/or Ohai/Chef in general.