I work a little with press releases (mind you, I'm not a legal worker of any type), and generally press releases are presented to be put out... So if you post them exactly (including appropriate author documentation) I would imagine you are okay.
As for material from sites such as cnn.com and the like. A few years ago I found an article about referees from cnnsi.com that I wanted to post on my site (was only a link). I contacted them about copyright issues and basically was told
1) They don't have e-mail
2) They don't have phones
3) They don't have faxes
4) Please mail your inquiery to XYZ and wait for a response (the cnn web-staff did not have any authority over copyrights and reproduction).
To make a long story short, you will probably be safe in that instance if you paraphrase, make it explicit (e.g. "To Summarize this news story"), and provide a link to the actual story, it seems like that would be safe. If you're already summarizing how much more work is adding a link?
But to add again, I don't have any legal training, just a little bit of antedoctal experience.