One of the biggest things that has set Android apart from iOS all these years is its openness - and that alone is a major reason that many who seek out free (as in libre) software have appreciated it. In fact, this could be one of the biggest reasons Android got off the ground so quickly, as I vividly remember around the time all of the excitement surrounding its focus on freedom. Sure - there have been other "free" mobile solutions, but few have succeeded in any real way. For all intents and purposes, Google seemed to have the right idea.
However, things have just changed. Android's SDK - everything required to rebuild the OS - is now considered non-free software. This means the software cannot be copied, modified, adapted, decompiled, reverse engineered, disassembled or forked. It remains "free" in the sense that you'll still be able to download the SDK to use and view, but modifying it in any way is out of the question.
Admittedly, this change will affect few people, although at least one fork of the SDK did in fact exist (called Replicant). However, the bigger issue here is that Google went back on its willingness to remain open, and adds yet another blemish to its infamous "Don't Be Evil" stance. With this move, those who seek out completely open software are left in a bad place, however it could bode well for Canonical's upcoming phone OS, based on Ubuntu.