Aerial IED
By Just read an interesting report in Defense News about the proliferation of aerial IEDs in Iraq. Apparently, insurgents are developing IEDs that can propel themselves into the air and explode. This has damaged several helicopters. Since US helicopters have been flying a very low altitudes since 2003 or so, the tactics may turn out to be quite effective.
No link, unfortunately.
UPDATE: Dave helpfully provides links here, and here.