You can do it as a functions plugin:
http://www.doitwithwp.com/create-functions-plugin/
http://justintadlock.com/archives/2011/02/02/creating-a-custom-functions-plugin-for-end-users
You can do it as a functions plugin:
http://www.doitwithwp.com/create-functions-plugin/
http://justintadlock.com/archives/2011/02/02/creating-a-custom-functions-plugin-for-end-users