WP-AutoBlog is a plugin for WordPress that inserts posts based on RSS feeds. It has been used to generate a single WP site containing all the posts of a WPMU install to provide cross-site search / tagging / categories, etc. It could also power a planet.
FeedWordPress is a plugin that inserts posts into a WordPress site from other RSS feeds. It can be used to create a planet (rss aggregator) on WordPress.
The Planet PHP FAQ from the authors including how to download the source they use.