This is a tricky question because in the end a press release is a press release. It serves one purpose. You are trying to win the attention of news editors in order to get them to write a story about you. Is there a different strategy for online press releases?
Yes and no.
You'd write the press release for online distribution pretty much the same way you'd write it for off line distribution except for one big difference. It's called SEO. Search engine optimization.
Your press release will get a lot more attention if it is written as much for the search engine audience as it is for the news audience. That means you want it to rank well for specific key phrases that people (news professionals) will search for.
Also, you must realize that your press releases published online have the potential to be read by members of the general public so you really have three audiences: 1) Search engines; 2) News professionals; 3) The general public. As long as you address the needs of your three audiences, your press releases will do their jobs well.