How to get list of joomla categories in 1.6 & 1.7

I can get a list of categories in joomla 1.5, but the same method isn't working for 1.6, 1.7. I have a plugin that needs to be able to retrieve a list of all categories in joomla version 1.6+ - how do I do it? The docs to joomla are very unhelpful, I would really appre开发者_JS百科ciate it if someone could provide a short php snippet that just gets an array of the categories.