[dTal]

It's not that there's a law (that I'm aware of) forbidding baseband software where the source is public - it's that operating a device with uncertified firmware is illegal. That means that devices with user-modifiable baseband firmware are also illegal (or at any rate, no manufacturer will take responsibility). So open community development of baseband firmware, in the usual model, is impossible - you can't legally test it (at least not without a questionably-legal SDR, a femtocell, and a faraday cage) and you can't certify it.

So while it might, technically, be true to say that open source basebands aren't illegal per se, the fact remains that they are functionally impossible because of the law.