I also noticed that Google count well links from press release, trusted blog, good appearence in social bookmark sites (front page or saved by many users).
That's why it is important once in a while to pay for press release or pr7 blog PPP.
If your site has 0 chance to appear in "Boston Globe Official site" or "Discovery Channel News"... don't miss a chance to pay once in year for pr7 blog or press release for $100-200 in local official newspaper.
As example i can give you a website which used to have great positions when "New York Times" wrote article about this them.
http://www.everynote.com/NYTimes.htm
They have only 100 backlinks and they was compeeting with websites who had 1000s backlinks... When article became too old and was placed in "New York Post" archieve which richable only for registered / subscribed users... website dropped back on positions they deserved with only 100 backlinks.