the current thinking about press releases as it relates to SEO is that they must be newsworthy to have any positive impact. my question is "what is the definition of newsworthy?" my company has published press releases on the web regarding things like new product lines and new business partnerships. are those types of events considered newsworthy?
I wouldn't know as it relates to SEO, if that is your question it would be better posed in the Internet Marketing forum. However, if you are really concerned with what newsworthy is, what you mentioned are both what I would consider newsworthy items. Also newsworthy are employee promotions, company events like anniversaries, seminars, etc. You can make just about any new thing in your company newsworthy. It isn't so much what you say as how you say it.
A newsworthy press release would mean that the press release which you get published for SEO purpose, should be worthy enough to be read and circulated around. If people aren't really interested to read your press release, then your credibility and the company's credibility would be at stake. And the end of the day, your marketing strategy wouldn't serve the purpose.
If you're looking for an exact definition of what defines a press release to be newsworthy, I don't know if there's a way for us to give you one. If you find that you don't really have a lot meaningful to announce in the press release, it may not be worthy.
It comes down to having something meaningful to say; if you're simply writing a press release a method of advertisement or to improve SEO, then it probably is not newsworthy.