I don't have time to catch up on the whole thread yet, but https://neo900.org/ should be interesting. The older n900 phone is quite open as well. Both [will] run an operating system called Maemo.
The neo900 solves some of the issues in current phones like the modem sharing memory space with the main CPU. I am highly interested in one myself, but don't think I'll be able to get an order in... hopefully another run happens :) Beware that the device OS may not be shipped in a completely working state. Some assembly required. As for how to make the device usable: I had been trying for some time to get Android running under KVM. Currently I have an ARM board which has a similar chip to the one my phone has. Sadly I can't figure out some bootloader and driver shenanigans so it doesn't seem like that will ever work. The neo900's processor *might* have what is needed to do that efficiently - ARM TrustZone - and if it does I was going to try it if/when I get one, as a replacement for Maemo (which is great and all but doesn't have such a large userbase).
