Telefonica in Brazil is one of the first carriers to distribute it, but I believe unlocked phones will be very common.. plus I've heard you could get an Android device and reflash it with B2G (it's Linux based and should work on the same devices as Android)
I really want to switch to this even if it is only 70% as good as Android on my GNex.
I've always been a Windows guy, never cared about free software and such, because it seemed like it was happening without too much fuss, but then, boom. Locked down phones, tablets, etc become the norm. And if our current phones become our future computing devices we are fucked.
I'm really glad to hear this argument for open source, and doubly so to know you plan on voting with your money. I'm looking forward to the Firefox OS, I hope it gets a ton of traction. I'm pretty sure I'll be getting one too. Cheers.