Diff from standard ec2.rb of Ohai 0.5.6

diff --git a/lib/ohai/plugins/ec2.rb b/lib/ohai/plugins/ec2.rb
index efefc6b..01ab5e5 100644
--- a/lib/ohai/plugins/ec2.rb
+++ b/lib/ohai/plugins/ec2.rb
@@ -53,16 +53,37 @@ def can_metadata_connect?(addr, port, timeout=2)
     end
   rescue SystemCallError
   end
-
+  Ohai::Log.debug("can_metadata_connect? == #{connected}")
   connected
 end
 
+def get_mac_address(addresses)
+  detected_addresses = addresses.detect { |address, keypair| keypair == {"family"=>"lladdr"} }
+  if detected_addresses
+    return detected_addresses.first
+  else
+    return ""
+  end
+end
+
+def has_euca_mac?
+  network[:interfaces].values.each do |iface|
+    has_mac = (get_mac_address(iface[:addresses]) =~ /^[dD]0:0[dD]:/)
+    Ohai::Log.debug("has_euca_mac? == #{!!has_mac}")
+    return true if has_mac
+  end
+  Ohai::Log.debug("has_euca_mac? == false")
+end
+
 def has_ec2_mac?
   network[:interfaces].values.each do |iface|
     unless iface[:arp].nil?
-      return true if iface[:arp].value?("fe:ff:ff:ff:ff:ff")
+      has_mac = iface[:arp].value?("fe:ff:ff:ff:ff:ff")
+      Ohai::Log.debug("has_ec2_mac? == true")
+      return true if has_mac
     end
   end
+  Ohai::Log.debug("has_ec2_mac? == false")
   false
 end
 
@@ -94,12 +115,15 @@ end
 def looks_like_ec2?
   # Try non-blocking connect so we don't "block" if 
   # the Xen environment is *not* EC2
-  has_ec2_mac? && can_metadata_connect?(EC2_METADATA_ADDR,80)
+  (has_ec2_mac? || has_euca_mac?) && can_metadata_connect?(EC2_METADATA_ADDR,80)
 end
 
 if looks_like_ec2?
+  Ohai::Log.debug("looks_like_ec2? == true")
   ec2 Mash.new
   self.metadata
   self.userdata
+else
+  Ohai::Log.debug("looks_like_ec2? == false")
 end
 

Ohai Fix for Eucalyptus ec2.rb

#
# Author:: Tim Dysinger (<tim@dysinger.net>)
# Author:: Benjamin Black (<bb@opscode.com>)
# Author:: Christopher Brown (<cb@opscode.com>)
# Copyright:: Copyright (c) 2009 Opscode, Inc.
# License:: Apache License, Version 2.0
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

provides "ec2"

require 'open-uri'
require 'socket'

require_plugin "hostname"
require_plugin "kernel"
require_plugin "network"

EC2_METADATA_ADDR = "169.254.169.254" unless defined?(EC2_METADATA_ADDR)
EC2_METADATA_URL = "http://#{EC2_METADATA_ADDR}/2008-02-01/meta-data" unless defined?(EC2_METADATA_URL)
EC2_USERDATA_URL = "http://#{EC2_METADATA_ADDR}/2008-02-01/user-data" unless defined?(EC2_USERDATA_URL)
EC2_ARRAY_VALUES = %w(security-groups)

def can_metadata_connect?(addr, port, timeout=2)
  t = Socket.new(Socket::Constants::AF_INET, Socket::Constants::SOCK_STREAM, 0)
  saddr = Socket.pack_sockaddr_in(port, addr)
  connected = false

  begin
    t.connect_nonblock(saddr)    
  rescue Errno::EINPROGRESS
    r,w,e = IO::select(nil,[t],nil,timeout)
    if !w.nil?
      connected = true
    else
      begin
        t.connect_nonblock(saddr)
      rescue Errno::EISCONN
        t.close
        connected = true
      rescue SystemCallError
      end
    end
  rescue SystemCallError
  end
  Ohai::Log.debug("can_metadata_connect? == #{connected}")
  connected
end

def get_mac_address(addresses)
  detected_addresses = addresses.detect { |address, keypair| keypair == {"family"=>"lladdr"} }
  if detected_addresses
    return detected_addresses.first
  else
    return ""
  end
end

def has_euca_mac?
  network[:interfaces].values.each do |iface|
    has_mac = (get_mac_address(iface[:addresses]) =~ /^[dD]0:0[dD]:/)
    Ohai::Log.debug("has_euca_mac? == #{!!has_mac}")
    return true if has_mac
  end
  Ohai::Log.debug("has_euca_mac? == false")
end

def has_ec2_mac?
  network[:interfaces].values.each do |iface|
    unless iface[:arp].nil?
      has_mac = iface[:arp].value?("fe:ff:ff:ff:ff:ff")
      Ohai::Log.debug("has_ec2_mac? == true")
      return true if has_mac
    end
  end
  Ohai::Log.debug("has_ec2_mac? == false")
  false
end

def metadata(id='')
  OpenURI.open_uri("#{EC2_METADATA_URL}/#{id}").read.split("\n").each do |o|
    key = "#{id}#{o.gsub(/\=.*$/, '/')}"
    if key[-1..-1] != '/'
      ec2[key.gsub(/\-|\//, '_').to_sym] = 
        if EC2_ARRAY_VALUES.include? key
          OpenURI.open_uri("#{EC2_METADATA_URL}/#{key}").read.split("\n")
        else
          OpenURI.open_uri("#{EC2_METADATA_URL}/#{key}").read
        end
    else
      metadata(key)
    end
  end
end

def userdata()
  ec2[:userdata] = nil
  # assumes the only expected error is the 404 if there's no user-data
  begin
    ec2[:userdata] = OpenURI.open_uri("#{EC2_USERDATA_URL}/").read
  rescue OpenURI::HTTPError
  end
end

def looks_like_ec2?
  # Try non-blocking connect so we don't "block" if 
  # the Xen environment is *not* EC2
  (has_ec2_mac? || has_euca_mac?) && can_metadata_connect?(EC2_METADATA_ADDR,80)
end

if looks_like_ec2?
  Ohai::Log.debug("looks_like_ec2? == true")
  ec2 Mash.new
  self.metadata
  self.userdata
else
  Ohai::Log.debug("looks_like_ec2? == false")
end

Eucalyptus says

Eucalyptus creates its instances with MAC addresses starting with d0:0d.
So I added a has_euca_mac? method that checks if any of the MAC addresses associated with the instance meets that criteria and added that as a check for looks_like_ec2?.

That seems to make things work