It's not open source, of course, but I find the gathering momentum toward opening up APIs in Web 2.0 applications to be an interesting spin on the "offline" open source world. First it was Facebook, and now it's LinkedIn. In the web world, it's not source code that gets opened (though these properties could do this and, in my mind, should), but rather APIs.
As to why companies are opening up the web, it has nothing to do with charity. It's actually very similar to the offline software world where you can put a lot of pressure on your closed competitors by opening up the value they want closed. Glyn Moody captures this well:
[O]nce somebody in a space starts opening up, its competitors simply have no choice but to follow if they want to keep the developers with them.
I don't view this as the end of LinkedIn's "open source" road, but rather the beginning. There is much more to gain by opening up than by closing off one's value.