I remember hearing about software radio in the context of open spectrum [brief notes], but didn't realise how far it'd come now: GNU Radio appears pretty established. Alex Laurie tells me that SDR is the next big thing in mobile--which is staggering, because it means that anything with a chip and knowledge of the correct protocols will be able to take part in the cellphone network, at least at the edges.
But also, it makes me think. If we have a country full of deployed SDR phones, that means they can tune to different frequencies easily, and there's also a lot of distributed computational power, and a network connection... Could the mobiles spend their downtime monitoring a different frequency, say, the cosmic microwave background radiation? The phones would comprise an extremely vast radio interferometer, a huge, distributed telescope. It'd be collaborative COBE and, by listening on other frequencies, a live, end-to-end, mobile SETI@home. Why download the radio signals when you can collect it?