The Replicant project was covered by Computer World today, with an interview of one of the main developers Paul Kocialkowski. The article was also picked up by Slashdot, where the mod was more cynical. Kocialkowski talked about the importance of free hardware and software when communicating in an insecure environment, and the difficulty of implementing an alternative when many of the drivers and firmware for key hardware is proprietary and secret.
Currently, Replicant is only supported by a few phones, including HTC Dream, Nexus One and Nexus S. The last one is probably interesting, as it is still a reasonably modern phone, can run Android 4.0, and have "factory binaries" available so you can start from scratch if you get stuck.
However, as pointed out in the Slashdot comment above, this project will always trail Google's releases. And as we've seen previously, the source codes does not always follow the release of the OS. Furthermore, Android in itself is not the most exciting OS around. The MeeGo (and presumably Tizen) initiatives are more standard GNU/Linux distributions from the get-go. Including features we've taken for granted over the last ten years or more: dependency resolving, updates over repositories, GNU applications. Of course, these will struggle with exactly the same problem: the proprietary drivers.