A Chef knife.rb that uses the git gem to determine which git branch you're in and then updates the Chef server endpoint accordingly.

knife.rb

require 'rubygems'
require 'git'

g = Git.open('./')

current_dir = File.dirname(__FILE__)
log_level                :info
log_location             STDOUT
node_name                'myworkstation'
cache_type               'BasicFile'
cache_options( :path => "#{ENV['HOME']}/.chef/checksums" )
cookbook_path            ["#{current_dir}/../cookbooks"]


if g.current_branch == "production"
  puts "Using Production"
  chef_server_url          "https://api.opscode.com/organizations/myproduction"
  validation_client_name   'mychef-validator'
  validation_key           "#{ENV['HOME']}/.chef/myproduction-validator.pem"
  client_key               "#{ENV['HOME']}/.chef/myproduction.pem"
elsif g.current_branch == "staging"
  puts "Using Staging"
  chef_server_url          "https://api.opscode.com/organizations/mystaging"
  validation_client_name   'mychef-validator'
  validation_key           "#{ENV['HOME']}/.chef/mystaging-validator.pem"
  client_key               "#{ENV['HOME']}/.chef/mystaging.pem"
else
  puts "Using Development"
  chef_server_url          "https://api.opscode.com/organizations/mydevelopment"
  validation_client_name   'mychef-validator'
  validation_key           "#{ENV['HOME']}/.chef/mydevelopment-validator.pem"
  client_key               "#{ENV['HOME']}/.chef/mydevelopment.pem"
end