John Robb mentions a feature at Adrian Holovaty’s weblog — customizable RSS feeds (unfortunately, as I write this, Adrian’s site is unreachable).
This is a feature which my weblog has had for several months. All you need to do is use this URL:
http://brian.carnell.com/index/rss/?body=
and add the text you want to look for after the “body=”
For example,
http://brian.carnell.com/index/rss/?body=John+Robb
. . . will return all of the weblog posts featuring Robb.
Personally, though, text-based searching like that quickly runs into some problems and limitations which is why I’ve got almost 250 topical RSS feeds (and growing) that can be subscribed to here.
A complete list of all the RSS feeds for this site can be found here.