Validação de números celulares no Brasil (válido em 1º de setembro de 2013).

gistfile1.rb

  # first 2 digits represent dialing code
  #   dialing codes in Brazil don't start with 0
  #
  # the following represent the phone
  #   must have 8 digits
  #     might have 9 if dialing code is 11, 12, 13, 14, 15, 16, 17, 18 or 19 
  #     http://g1.globo.com/sao-paulo/noticia/2013/08/nono-digito-comeca-valer-para-telefones-celulares-do-interior-paulista.html
  #   first digit must start with 7, 8 or 9
  #     might start with 5 as well if dialing code is 11
  # 
  MOBILE_REGEXP = /\A(11(5|[7-9])\d{7,8}|1[2-9][7-9]\d{7,8}|[2-9]\d[7-9]\d{7})\z/

E para quem procura um validator completo para Rails que também avalia números fixos: https://gist.github.com/leods92/6405552