Google Groups and Google Web Search are the only search engines from the ones mentioned above that don't provide feeds for the results. In fact, Google is the only major search engine that doesn't offer feeds for search results.
The new feature from Google Alerts is useful, but Google should've provided an option to subscribe to feeds for each search result. Right now, the feeds from Google Alerts have cryptic addresses like:
http://www.google.com/alerts/feeds/LONG_NUMBER/ANOTHER_LONG_NUMBER
and you can only generate feeds from Google Alerts.
Note that you have to log in to a Google account if you want to get feeds for your alerts.