Okay, Feedgit is a good way of quickly creating search-based feeds from news sources. But if you really want to catch online mentions of a particular topic from a whole ton of sources, check out MonitorThis.
With MonitorThis you can subscribe to 22 different search engine feeds at the same time. Enter a search term and click the ‘make monitor.opml’ button to get a list of rss feeds in OPML format.
Just plug in your search term and copy the output, paste it into notepad and save the file as monitor.opml, then import into your favorite aggregator.
My only complaint is that I get a LOT of duplicate hits. I’d like to load this OPML into Yahoo Pipes and filter for duplicates, but (the otherwise excellent) Yahoo Pipes doesn’t import OPML yet (dagnabbit).