I have the need to publish category specific feeds for various reason. For example, I publish feeds from just the "Home" category to houseblogs.net, so they don't get a lot of non-houseblogging related items. I use Yahoo pipes to create a new rss feed with just this category information.

The pipe was pretty simple to make. I use my RSS feed as the source, filter based on the item.category XML item and use the output as the new rss feed. I have published this pipe and can be viewed at this link: http://pipes.yahoo.com/pipes/pipe.info?_id=ZuZMORNV3BG_9SduLe2fWQ.