gem.list

activemodel (3.0.0.beta4)
activesupport (3.0.0.beta4)
bson (1.0.3)
builder (2.1.2)
i18n (0.4.1)
mongo (1.0.3)
mongoid (2.0.0.beta7)
rake (0.8.7)
rdoc (2.5.8)
tzinfo (0.3.22)
will_paginate (3.0.pre)

output.1.8.7-p249.txt

========Before mongoid
Respond to asc - false: undefined method `asc' for :sym:Symbol
Respond to ascending - false: undefined method `ascending' for :sym:Symbol
Respond to desc - false: undefined method `desc' for :sym:Symbol
Respond to descending - false: undefined method `descending' for :sym:Symbol
Respond to gt - false: undefined method `gt' for :sym:Symbol
Respond to lt - false: undefined method `lt' for :sym:Symbol
Respond to gte - false: undefined method `gte' for :sym:Symbol
Respond to lte - false: undefined method `lte' for :sym:Symbol
Respond to ne - false: undefined method `ne' for :sym:Symbol
Respond to near - false: undefined method `near' for :sym:Symbol
Respond to in - false: undefined method `in' for :sym:Symbol
Respond to nin - false: undefined method `nin' for :sym:Symbol
Respond to mod - false: undefined method `mod' for :sym:Symbol
Respond to all - false: undefined method `all' for :sym:Symbol
Respond to size - false: undefined method `size' for :sym:Symbol
Respond to exists - false: undefined method `exists' for :sym:Symbol
Respond to within - false: undefined method `within' for :sym:Symbol
========After mongoid
Respond to asc - false: undefined method `asc' for :sym:Symbol
Respond to ascending - false: undefined method `ascending' for :sym:Symbol
Respond to desc - false: undefined method `desc' for :sym:Symbol
Respond to descending - false: undefined method `descending' for :sym:Symbol
Respond to gt - true: #<Mongoid::Criterion::Complex:0xb73c52a0 @operator="gt", @key=:sym>
Respond to lt - true: #<Mongoid::Criterion::Complex:0xb73c51c4 @operator="lt", @key=:sym>
Respond to gte - true: #<Mongoid::Criterion::Complex:0xb73c50fc @operator="gte", @key=:sym>
Respond to lte - true: #<Mongoid::Criterion::Complex:0xb73c5034 @operator="lte", @key=:sym>
Respond to ne - true: #<Mongoid::Criterion::Complex:0xb73c4f6c @operator="ne", @key=:sym>
Respond to near - true: #<Mongoid::Criterion::Complex:0xb73c4ea4 @operator="near", @key=:sym>
Respond to in - true: #<Mongoid::Criterion::Complex:0xb73c4ddc @operator="in", @key=:sym>
Respond to nin - true: #<Mongoid::Criterion::Complex:0xb73c4d14 @operator="nin", @key=:sym>
Respond to mod - true: #<Mongoid::Criterion::Complex:0xb73c4c4c @operator="mod", @key=:sym>
Respond to all - true: #<Mongoid::Criterion::Complex:0xb73c4b84 @operator="all", @key=:sym>
Respond to size - true: #<Mongoid::Criterion::Complex:0xb73c4abc @operator="size", @key=:sym>
Respond to exists - true: #<Mongoid::Criterion::Complex:0xb73c49f4 @operator="exists", @key=:sym>
Respond to within - true: #<Mongoid::Criterion::Complex:0xb73c492c @operator="within", @key=:sym>

output.1.9.2-preview3.txt

========Before mongoid
Respond to asc - false: undefined method `asc' for :sym:Symbol
Respond to ascending - false: undefined method `ascending' for :sym:Symbol
Respond to desc - false: undefined method `desc' for :sym:Symbol
Respond to descending - false: undefined method `descending' for :sym:Symbol
Respond to gt - false: undefined method `gt' for :sym:Symbol
Respond to lt - false: undefined method `lt' for :sym:Symbol
Respond to gte - false: undefined method `gte' for :sym:Symbol
Respond to lte - false: undefined method `lte' for :sym:Symbol
Respond to ne - false: undefined method `ne' for :sym:Symbol
Respond to near - false: undefined method `near' for :sym:Symbol
Respond to in - false: undefined method `in' for :sym:Symbol
Respond to nin - false: undefined method `nin' for :sym:Symbol
Respond to mod - false: undefined method `mod' for :sym:Symbol
Respond to all - false: undefined method `all' for :sym:Symbol
Respond to size - true: 3
Respond to exists - false: undefined method `exists' for :sym:Symbol
Respond to within - false: undefined method `within' for :sym:Symbol
========After mongoid
Respond to asc - false: undefined method `asc' for :sym:Symbol
Respond to ascending - false: undefined method `ascending' for :sym:Symbol
Respond to desc - false: undefined method `desc' for :sym:Symbol
Respond to descending - false: undefined method `descending' for :sym:Symbol
Respond to gt - true: #<Mongoid::Criterion::Complex:0xa82bc20 @key=:sym, @operator="gt">
Respond to lt - true: #<Mongoid::Criterion::Complex:0xa82bb1c @key=:sym, @operator="lt">
Respond to gte - true: #<Mongoid::Criterion::Complex:0xa82ba2c @key=:sym, @operator="gte">
Respond to lte - true: #<Mongoid::Criterion::Complex:0xa82b93c @key=:sym, @operator="lte">
Respond to ne - true: #<Mongoid::Criterion::Complex:0xa82b84c @key=:sym, @operator="ne">
Respond to near - true: #<Mongoid::Criterion::Complex:0xa82b75c @key=:sym, @operator="near">
Respond to in - true: #<Mongoid::Criterion::Complex:0xa82b66c @key=:sym, @operator="in">
Respond to nin - true: #<Mongoid::Criterion::Complex:0xa82b57c @key=:sym, @operator="nin">
Respond to mod - true: #<Mongoid::Criterion::Complex:0xa82b48c @key=:sym, @operator="mod">
Respond to all - true: #<Mongoid::Criterion::Complex:0xa82b39c @key=:sym, @operator="all">
Respond to size - true: #<Mongoid::Criterion::Complex:0xa82b2ac @key=:sym, @operator="size">
Respond to exists - true: #<Mongoid::Criterion::Complex:0xa82b1bc @key=:sym, @operator="exists">
Respond to within - true: #<Mongoid::Criterion::Complex:0xa82b0cc @key=:sym, @operator="within">

test.rb

require 'rubygems'
@map = %w{asc  ascending  desc  descending  gt  lt  gte 
     lte  ne  near  in  nin  mod  all  size  exists 
     within}
def dump()
  @map.each do |op|
    print("Respond to #{op} - #{:sym.respond_to?(op.to_sym, true)}: ", 
      begin
        :sym.send(op.to_sym).inspect
      rescue 
        $!
      end, "\n")
  end  
end
print "="*8+"Before mongoid\n"
dump()
print "="*8+"After mongoid\n"
require 'mongoid'
dump()

The sorting symbol methods are in master, not in beta7 gem.

http://github.com/durran/mongoid/commit/e8490391050aeeb3963fe7b3b3df8142424464a4

Got it! The right commit is http://github.com/durran/mongoid/commit/9cb502ae167e6d2d1da0ee128c07d3cdaf86be06 though.
It's about symbol extension. Seems that commit is made one day after gem was released.
Will use git master as a source.