Totally untested, just forked and tweaked for discussion

uri_validator.rb

require 'addressable/uri'

#Accepts options[:message] and options[:allowed_protocols]
class UriValidator < ActiveModel::EachValidator
 
  def validate_each(record, attribute, value)
    uri = parse_uri(value)
    if !uri
      record.errors[attribute] << generic_failure_message
    elsif !allowed_protocols.include?(uri.scheme)
      record.errors[attribute] << "must begin with #{allowed_protocols_humanized}"
    end
  end

private
  
  def generic_failure_message
    options[:message] || "is an invalid URL"
  end
  
  def allowed_protocols_humanized
    allowed_protocols.to_sentence(:two_words_connector => 'or')
  end
  
  def allowed_protocols
    @allowed_protocols ||= ([options[:allowed_protocols]].flatten || ['http', 'https'])
  end

  def parse_uri
    Addressable::URI.parse(value)
  rescue URI::InvalidURIError
  end

end

uri_validator_spec.rb

describe UriValidator do
  subject do
    Class.new do
      include ActiveModel::Validations
      validates_with UriValidator
      attr_accessor :url
    end.new
  end
  
  it "should be valid for a valid http url" do
    subject.url = 'http://www.google.com'
    subject.should be_valid
  end
end

Line 29 of uri_validator.rb should be:

def parse_uri(value)

UriValidator#allowed_protocols should be:

def allowed_protocols
  @allowed_protocols ||= [(options[:allowed_protocols] || ['http', 'https'])].flatten
end