We've done a small update to the Steam build, to address an issue with the Launcher app. This doesn't affect the game itself, but there is a small update out, so in case you're wondering why your game was updated, that was it.
The KSP Launcher shows you a news feed so you've now got a place to quickly check out the latest news and dev blogs, and it also lets you tweak game settings before opening the game, which means you now have a way to undo any settings that might prevent the game from working properly, plus new settings which are only available from outside the game, like frameless windowed mode. Check out the Advanced Tab on the launcher settings to see the new options.
We've reverted the app config, to not open the launcher app just yet, since there are still a few issues with it on OSX and Linux, and also with the Steam Overlay. The Launcher is still available on your game files, so you can use it already if you want to (to use advanced settings and such), but Steam will open KSP directly for the time being, until we are 100% sure the launcher is functioning as it should.