It's great to have a coarse-grained sanity check from a trusted source, but I don't think this is really a plausible replacement for NTP (not that you need NTP when you have a mobile baseband and GPS).
I've heard that the various GSM / UMTS / etc encryption and authentication schemes are not at all secure (like WEP bad). So if mobile baseband time relies on that, it's not particularly secure. It may be less convenient to MITM though, it's hard for non-law-enforcement to get a Stingray, and getting GnuRadio working for this would be far from plug-and-play.