No matter how configurable you make the CSS for your plugin, you’ll never be able to take into account all the layout’s, colour schemes and user preferences out there.
@Paul - I agree that there is no universal approach to css. However sites like csszengarden.com prove that if developers are mindful of the markup they produce, there are a surprising amount of layouts which can be re-skinned through CSS alone (or the occasional bit of jQuery magic :)

I use the plugins.css file approach. It would be nice to see some standardization there, but I’m not sure it will/could happen.
@chip - What is the plugin that you have wrote which uses that look-for-a-plugin-centric-css-file-in-active-theme approach? The "standardization" that is starting to come together for child themes only came about by people talking about it and - where possible - taking their own steps towards implementing it as you have done. As the community shows interest in various areas, the core development team has done a great job of extending / supporting these visions for the platform.

Anyone else have experience with plugins (either developing or using) which look to an active theme css file before loading their own?