The US is going to designate Iran's Revolutionary Guards force as a terrorist group. The unit is an official military group in Iran, and it'll be the first time such a unit has ever been listed as a terrorist force by the US.
There are two main reasons why the Americans are doing this. The main impact of the designation is that it'll allow the US to target the financing of the Revolutionary Guards. Washington hopes that'll affect the Guards' ability to supply and train the various insurgent groups operating in Iraq and, increasingly, Afghanistan. Iran shares large and porous borders with both countries, and has long been accused of helping fighters attack American and British troops.
The second main reason for targeting the Guards, is to try to up the pressure on Tehran in the standoff over Iran's nuclear ambitions. The threat of a military strike by the US on Iran has receded over the past year, as they've followed a course of diplomacy to try to resolve the dispute. This action against the Guards is a sign of Washington resuming a tougher stance. Although there's no question of the lame-duck and weak Bush administration starting a war against Iran now, this move makes it slightly more likely the next occupant of the White House could choose that option.