Here's a style-switcher that uses the DOM. It imports the styles as normal to begin with but when the script comes into play it removes the style element and puts in a link element. This is for two reasons, firstly IE won't let me append ANYTHING to a style element (v. sucky) and secondly a link element is just easier to work with. :p
Don't worry about using link elements by the way, if a browser understands the DOM then it will also understand the import rule, so even though I'm using <link>, NN4 and IE4 will be cut out the loop and you won't have any half-rendered styles.
OK, here's the code, I've commented it so you shouldn't have much trouble working out what bits do. If you do have a spot of bother though I'll be happy to help.