Software Radio: An Engineer's Dream and a Regulator's Nightmare
One interesting development in recent years is the software radio, with projects like GNU Radio producing open source implementations. Of course, a radio can't be entirely software, but the hardware required is relatively simple—little more than an antenna and a DAC/ADC process.
In the past, it was easy to enforce regulatory compliance for radio devices. You checked that the hardware was limited to operating at certain frequencies and power levels; if it was, then you allowed it to be mass-produced.
For software radio, the power levels and the frequency are controlled in software. This is problematic for regulators, especially when combined with open source drivers, since it means that the end user can easily turn a legal device into an illegal one. (Of course, this was always possible by adding amplifiers and larger antennae, but it's a lot easier if you just need to download patched firmware.)
While it doesn't make regulators happy, software radio does present some interesting opportunities for communications. When you can use the same hardware for HDTV and WiFi signals (at the same time, with a fast enough processor), there are a lot of possibilities. Pure software radio is incredibly processor-intensive, and therefore doesn't make sense for mobile devices (where dedicated silicon can consume less power), but it provides some inspiration. Dynamically-configurable signal-processing elements can be integrated into the same package, and you can now get low-power chips that can run a number of different wireless protocols.
Devices in the next couple of years are likely to support WiFi, WiMAX, HSPDA, GSM, and any other protocols that become popular with very little power cost and the ability to switch rapidly between them.
While this change isn't entirely useful on its own, it becomes a lot more exciting when combined with Mobile IPv6—giving your device the capability of hopping between a variety of protocols, picking the cheapest one for whatever you're doing.