A cellular base station transmitter using SDR has been approved by the FCC: The FCC press release says this is the first SDR device they have approved, and is working to streamline the approval process. SDR uncouples the notion of what frequencies a device can receive and transmit from hardware and allows software control (within a variety of parameters). SDR could vastly reduce costs and increase flexibility of devices that could be used in more countries and across more bands without expensive custom hardware.
Oddly, I thought SDR was in wide use. For instance, Atheros has based its products on SDR, locking down the abilities of their device to work in specific frequencies. The madwifi drivers for Atheros use a Hardware Abstraction Layer controlled by the open-source project manager--and not part of the source code of the project--to allow Linux and other drivers that cannot manipulate the frequencies used by the chips.
Also oddly, I cannot find a single bit of information outside of the madwifi project that indicates that Atheros uses SDR.