Chromebook users are getting a stable channel update, Chrome 21 is being updated to 21.0.1180.92, part of Chrome OS 2465.227.0. As you'd expect, it's a stability and bug-fixing update, nothing major. Still, it should eliminate some annoying problems Chromebook users have been dealing with.
There's just one major problem fixed in this update, a crash that affected the connection manager when resuming with the 3G connection enabled. Obviously, for WiFi only models, this is not an issue.
The update applies to the new cheap Chromebook, the Samsung Series 3, and it's the first update these devices are getting, depending on the version of Chrome they shipped with.
As always, the update should be completely invisible to most users, unless you obsessively monitor the Chrome/Chrome version or have been affected by the bug being fixed, you're unlikely to notice it, especially a minor one like this.