I have tried many different version of mongoid and I have never seen validates_uniqueness_of :field work for me. Does this work as advertised in the docs?
I am now on the latest version of mongoid (beta 19 ) and it still doesn't work, here is a sample script showing it doesn't work:

require 'rubygems'
require 'mongoid'

Mongoid.configure do |config|
name = "unique_test"
host = "localhost"
config.master = Mongo::Connection.new.db(name)

config.persist_in_safe_mode = false
end

class User
include Mongoid::Document
embeds_many :posts
field :login
end

class Post
include Mongoid::Document
field :data
field :sid, :type => Integer
embedded_in :user, :inverse_of => :posts
validates_uniqueness_of :sid
end

sid = 21088517421

u = User.create(:login => 'me')
u.posts << Post.new(:sid => sid,:data => 'data')
u.posts << Post.new(:sid => sid,:data => 'data')
u.save
u.reload
puts u.posts.size == 2 #WHY is this true?
puts u.posts.inspect
u.destroy