Born out of Podcast 30 Days project is the new service XML Stitch.
XML Stitch attempts to do what multiple other services have done in the past, combine RSS feeds. XML Stitch is able to take multiple, valid, RSS feeds and combine them into one feed, sorted by date, that will in turn validate. With XML Stitch you have the ability to create a combined feed for a certain period of time, say for the month of November for NaPodPoMo, or any other short-term project.
The service is still being setup. Look for an announcement in the early part of 2012.