Fog 0.5.3 was the last version to support Fog::AWS::EC2, Fog::Rackspace::Servers, etc. As of Fog 0.6.0 you must be using a version of Chef that includes this patch (currently 0.9.14.rc.1) or you will get an error similar to this:
`run': uninitialized constant Fog::Rackspace::Servers (NameError)
from /usr/lib/ruby/gems/1.8/gems/chef-0.9.8/lib/chef/knife.rb:127:in `run'
from /usr/bin/knife:19:in `load'
See also CHEF-2030 for the next round of Fog API changes.