Amazon has published developer details for the Kindle Fire. Most notably they support apps up to API level 10 (Gingerbread 2.3.4+).

This is fantastic news as it discounts all rumors that the device was running ancient versions of Android. That's important as it ensures the device comes out of the gate with the many foundational improvements that Google has added to the OS. It also amply demonstrates that Amazon's fork is minimalist, and is likely limited to the shell.

The hardware itself is missing a few features. It doesn't have a gyroscope, for instance, however such is a hardware facet that has gone almost entirely unused in the few products that feature it (some are confusing a gyroscope for an accelerometer. The device does have full motion sensing, just not of the gyroscope variety). The target itself is generally locked down, but that's okay -- for Android developers you can choose to participate in that curated garden or not, with ample alternatives if you decide not to.