There are several reports over at XDA Forums that this update also breaks one of the rooting tools, SuperOneClick. Needless to say, that's not good. But it's also not a major concern because I expect this update to be hacked momentarily.
BTW, it's not all bad news. I'm also following a thread over at the discussion boards on Amazon. User reports are also coming in, and some are seeing improvements in performance as well as changes in how the KF functions. Books can now be removed from Carousel, the slide movement is now smoother, and there's a new "Restrictions" row added to settings.
If you haven't already rooted your KF, now might be a good time to turn off the Wifi and go run the Kindle Fire Utility I posted about this morning. Root your tablet with zergrush, and then install TWRP. That's a recovery and updater app that resides on your KF and makes it easy to install and manage updates.
Once you have TWRP installed, you can use it to install Cyanogen Mod, which you can find over at XDA Forums. And once you have CM running, you're safe from Amazon borking with your tablet.
But do you know what? If you haven't hacked your KF, just let the update happen. It sounds like it adds a number of useful features. Also, it's going to get hacked fairly quickly so this really isn't a major problem.
The update is being pushed out over the air, but manual install is out because it's not yet listed on the support page. But of you wait a few days, I'm sure it will be.