The group that maintains the RSS standard is exploring the addition of some taxonomy elements. (Found via Ease.)

At first glance it looks like it will give you the ability to add pointers to related information and/or topics on your own web site or elsewhere. Taking a weblog as an example, you could add category-specific archives links to individual posts in an RSS feed. A news reader could then render links to your category archive for a particular post which the user could then follow if they want to see whatelse you have said on the overall subject.