A few weekends ago I had the need to combine RSS feeds and was not satisfied with any of the solutions I found on the net.  A few hours later, I had a general proof of concept script working – which is now called RSS Fusion.  The site will fuse together multiple rss feeds and attempt to retain as much information as possible.  Currently it only works on RSS, and does not retain enclosures (podcasts) but I will look into adding that ability in the future.

Check it out: http://www.rssfusion.org

Merry Christmas!