The Wall Street Journal is online with an about persistent search from companies like PubSub. The article is titled: New Web-Watching Tools Pique Interest of Investors.
Providers of persistent-search services like to say it's "prospective," or forward-looking, rather than "retrospective," which is how they describe aggregators such as Yahoo and Google. It often takes most search engines days to add new content from sources such as blogs or corporate Web sites. Yahoo and Google scan billions of sites and download them into their massive computer databases, which can add a significant time lag between when content appears and when it is available for searches. Google does provide a service that will notify users of new Web content, but with less urgency when compared to persistent-search engines.
Btw, one of my favorite tools (one I couldn't live without) is called WebSite-Watcher (Windows only). This very stable shareware monitors both RSS feeds and just about ANY web page for changes. WSW then notifies you when changes occur (you define how often it looks for changes) and highlights the new or changed text. Think about all of the pages on the web where no RSS feed is available but you really need to check on a very regular basis. Sometimes the addition or removal of just a couple of words can signal something big about to happen. TrackEngine, is very similar if you want a web-based solution. Other services include:
We're also started to see targeted tools looking for new material in specific areas of interest. For example, DiplomacyMonitor.com, looks for new primary documents from governments around world and posts them onto a single page while at the same time caching the content and making it all searchable.