I came across FeedShake this morning:
Feedshake tool helps you to generate new feeds by merging, sorting and filtering existing online RSS feeds.
It has two modes allowing for various features including sorting, filter by keyword (inclusive and exclusive), re-titling, etc. Matt Siegler pointed me to this Lifehacker (profile) post, and Michiel Dethmers over on the backstage mailing list pointed me to another similar site: sux0r.
UPDATE: Eric Williams at backstage pointed to another mixer: aggrssive.