@scotland, I think of it like this. If the thing I am styling is a "type" of an object, and I think it could theoretically be reused multiple times on a page, then apply styles using a class. If, however, the thing I am styling is an "instance" of an object, and I know it explicitly refers to an element that will only ever appear once on the page, then use an id. Often times this means that the core structure of the page uses ids, and those contain various classed elements. For example:
The important distinction is that id's must be unique and only appear once on a page. If you think you're styling a more general "class" of elements, then use class.