Unlike some vendors which shall remain unnamed (*cough*, HTC, *cough*), Amazon didn't make us wait for the mandatory open source bits of the Android Fire's kernel and released them over at their Source Code page the same day the tablets themselves started arriving in consumers' hands. The download, which comes as a compressed tar.gz, weighs in at a whopping 809MB.
The source code should allow for custom ROMs and tweaks to the OS, which we can hopefully expect soon, considering the Fire has already been rooted. There is still no word on whether its bootloader can be easily unlocked, however, so theoretically there could be a few potential bumps on the road to the first custom ROMs. Time and some good old checking up on this xda forum will show soon enough - we'll definitely keep our eyes peeled.