At Carbon Games we're using Go for backend servers, including game sessions server. If you go and play AirMech multiplayer game right now, you'll be playing it on server written in Go.
It's platform agnostic in sense that it runs on all platforms where desktop version of Chrome browser runs. But if you mean Chrome is platform, then yes it runs only in Chrome for now.
It's not standard for all browsers, but I think that's going to change with more games like AirMech that wouldn't be possible to make in .js/WebGL/HTML5.
It's also fully open sourced with permissive license (BSD), which should not present any barrier for others to adopt it. Adoption will likely be decided by politics (f.e. http://www.theregister.co.uk/2010/06/24/jay_sullivan_on_fire...), rather than technical issues.
One interesting thing about CWS apps is that they can be ran in window mode, and shortcut can be placed on desktop. At that point NaCl app doesn't look any different than native desktop app. It's pretty cool!
