Now, what he did wasn't actually hacking the tablet, but it was the first step. He discovered how to get root access, which (if you're not familiar with Linux terms) is something akin to admin privileges on a Widows PC. This is the first thing that hackers need to before they bypass the manufacturer's restrictions and start installing and testing apps.
But it's still a great first step.It means we're going to see custom firmwares for the Kindle Fire a start showing up within the next week or so, and we could even see a few firmwares (that are safe for the end user to install) released within a month.
Speaking of the Kindle Fire, Amazon put a number of subtle restrictions on it. For example, the KF cannot see the Aldiko or Kobo reading apps in the Amazon Appstore. With other tablets that would mean that the tablet isn't compatible, but in this case Amazon doesn't want you to see the competition.
I got the Aldiko app elsewhere and it installed just fine. I tried both the older v1.2 and the new v2.x versions and both work okay. Since Amazon wouldn't let me see the version available via their app store, I can only conclude that they're hiding it from me.
In case you're interested, I downloaded one version from a website and I emailed the other version to myself and then downloaded it from the email. So don't worry; there are other ways to get apps than via Amazon.
Update: And just to be clear, I have not rooted my KF yet. I installed the apps on a KF running on the exact same firmware that Amazon shipped.