You can use them if you keep all of the information intact. You cannot change a word or leave of the credits. I would always ask a web site owner before using any content out of respect. They should have no problem with you posting a press release for them.
jrap - Webmaster World generally does not allow them to be posted due to copyright issues - you might want to check here [webmasterworld.com] for Brett's reasons. He's been burned before - learn from his experience :)
Many of the press releases I receive and view have tag marks saying that it can be restributed as long as it is in the same blah blah blah - the same. Do you see any of those kind of wording with them, if not simply contact the company and ask if it can be redistributed :)
jrap - I'm assuming you found this press release on the company's site that the press release is about? If this is the case, they may or may not have standard guidelines in place for press release publication. But press releases are written with the intent to spread a message, if you can be a catalyst to that, I'm sure they'll welcome it.
Why not send the site an email asking permission to post the release or set up a link to it ... while you're at it ask for a reciprocal link to your site if the content/site is appropriate.
Anything that is published is automatically covered by copyright unless the copyright owner explicitly puts it into the public domain (older stuff may already be in pub dom - the law is very specific with dates). Press releases are no exception. They are just a document which is copyrighted to which the owner has granted a license to copy. He still owns the copyright.