Google Calling: Inside Android, the gPhone SDK (O'ReillyNet)

Note that copyright only covers expression, not function. There is a bunch of recent case law
(in the US) that suggests that interfacing to a program is non-infringing.

This is all really gray area, both in terms of what constitutes a derived work of a piece of
software and in terms of what is allowed by fair use regardless. Coupling that with the terms
of the GPL that don't exactly align with copyright (using permission to distribute the covered
work as a lever to also control the way its distributed and what's distributed with it), it's
really hard to predict what would happen in a court test of issues like proprietary drivers.
It's best (and most polite to authors) to just avoid questionable practices.

Device manufacturers, in many cases, don't really care that much about exposing their drivers.
Their value-add is usually in more domain-specific areas, usually above the kernel.

I think the key points that handset manufacturers and carriers will want to lock down are
around (a) access to the air stack and RF control, (b) access to middleware services that use
the network (placing calls, sending messages), and (c) IP-control mechanisms (DRM, content
copying). (a) because it affects whether the FCC will let them ship the device, (b) because
they potentially can be used for DoS attacks on the network and can make users very unhappy
about unexpected charges, and (c) because they want to be able to offer access to content that
they can't get without guaranteeing (with substantial financial penalties) that their access
controls work. Their key GPL question will be around whether they can allow replacing the
kernel while still meeting those guarantees. Drivers are not that big a deal.