I can say with some confidence that an open source baseband will never happen. The carriers won't allow it, and there is good (technical) justification for that. Basically, it is easy to mess up some "tiny" detail of the standard, and disrupt the local network for all nearby users. The compliance testing needed for any phone is huge and expensive.
Unless I can't make a 3G equivalent to wireshark I'll have a hard time calling the platform open.