I just stumbled across DukaPress, what appears to be a fully fledged e-commerce plugin for WordPress.

DukaPress is a simple and free WordPress e-commerce system. It is open source. With DukaPress you can quickly and easily set up a fully featured online shop which can be used to sell digital or physical goods to customers all over the world.

That is no small claim. Shopping cart and e-commerce functionality have always been sorely lacking in WordPress. While there are a few shopping cart plugins out there, none are very robust. If DukaPress really is as good as it looks, then e-commerce for WordPress has finally arrived.