Danger Room has an awesomely comprehensive look at all the cell phone jammers the US military used during the Iraq War. Those jammers proved incredibly important in stripping insurgents of their most powerful weapon—the IED.
The US government has spent $17 billion buying 50,000 jammer units with fantastic names like Warlock Green, Warlock Red, Warlock Duke, Acorn, and more. But in the beginning of the war, there was a little bit of a cat and mouse game between radio-frequency jammers and IEDs and jammers were far behind. They were too slow, they couldn't adapt as well and they could only offer protection of only a few yards. Hell, occasionally two jammers would lock onto each other and cancel one another out.
But they got better. According to Danger Room:
The Navy sent to Iraq hundreds of electronic warfare specialists, to bring the cacophony produced by 14 kinds of jammers into some sort of harmony. Protocols were established, to allow one device to send its signal and then go silent for a few milliseconds, so another gadget could broadcast; that allowed Warlock Red and Warlock Green to be packaged into a single, combination unit...The intelligence specialists at the Combined Explosive Exploitation Cells got faster and faster at analyzing which frequencies the insurgents were using. That, in turn, allowed the jammers to be updated more quickly, so they could counter emerging threats.
A new generations of jammers were introduced too, they could cover a broad range of frequencies and perform specific "set-on" jamming which mean that "rather than confuse a receiver with a modified version of its own signal, Duke had a series of built-in jamming responses, designed to fool very specific devices." And as jammers got better, the insurgents in Iraq largely abandoned the use of IEDs and deaths from IEDs dropped.
However, IEDs are still a serious problem in the rest of the world so defense manufacturers are ramping up even newer technologies to combat them. It's basically going to be a brand new cat and mouse game. Read the entire excellent report at Wired's Danger Room.