<p>Because I run a blog aggregator website which checks a large list of RSS feeds for new posts every hour so I will be happy if its possible to use google feed api or <a href="http://code.google.com/apis/ajaxfeeds/" rel="nofollow noreferrer">Google AJAX Feed API</a> instead of making the cron jobs to read the whole feed to know if its updated or not.
like this <a href="http://getsatisfaction.com/rososo/topics/updates_dont_seem_to_be_dated_right" rel="nofollow noreferrer">link text</a></p>

You can use the Google Feed API, however instead of polling the feed source every hour you will still need to poll the google feed API every hour.  It doesn't notify you of feed updates.  If you want to be notified when to update feeds, then you should look into using a ping server,
http://en.wikipedia.org/wiki/Ping_blog
to determine when feeds are updated so that you know it is time to fetch the feed again.  Not all feeds use a ping server, but it might help you for those that do.