PURE SPECULATION, INFORMED BY NO INSIDE KNOWLEDGE, FOLLOWS:

The oFono project really introduces a "wild card" into the mobile phone world. The only way Nokia or the carriers are going to be able to control how the users use their phones is to do one of three things to the supposedly open source telephony stack:

1. Closed: Keep the modem plugins as binaries with closed APIs
2. Hobbled: Let the modem drivers be open source, but strip out any parts that you don't want developers to use (this might be in conjunction with 1, making some parts of the driver open and some parts closed)
3. Broken: The Nokia hardware, as released, is physically incapable of voice, or the source code is purposely misleading, or the portion of oFono being used in Maemo 5 is incompatible with the open source project in critical ways

If Nokia resists the urge to do those things (and we'll have to wait and see), and they don't officially support voice in Maemo 5, then I guarantee a swarm of community developers will descend on the Maemo devices and, if it is technically possible, they will have community support for voice working shortly after their release.

EDIT: I notice the diagram is gone. Perhaps because of the "plugin inside the gpl border" issue? Or maybe too much bandwidth?