Top attack missiles are simply new warheads on existing anti-tank missiles. As the name implies, the new warhead detonates when the missile flies over the tank, penetrating the thinner armor on top. These have been around for two decades, but have so far seen most action in a long string of successful tests.
SADARM, or "sensor fused munitions" are 1980s era weapons that are popped out of an artillery (or mortar) shell or artillery rocket, or a bomb container, and drift to ground, using a built in radar and heat detector to find a tank and then attack it with a top attack warhead. The Russians have a version of their own, MOTIV-3M, for sale to all with hard currency.
Just because these weapons have not been used in combat, doesn't mean they won't work. All these new weapons have passed test after test for decades. There haven't been that many tank battles in the last twenty years, and it's just a matter of time before the bad guys (say, Iraq) gets there hands on some of this stuff (if they haven't already) and uses it. The United States plans to counter these weapons by fighting a fast moving battle and using air power and artillery to destroy most enemy artillery before they can use SADARM or track busters. But eventually, most senior Pentagon officials believe, M-1s will encounter these weapon, and get knocked out.
And that is why American armored troops are having a hard time getting money for an M-1 replacement. The Pentagon may be wrong (wouldn't be the first time), but they are down on MBT survivability and looking for future alternatives.
While the U.S. currently has some 7,000 of the best, combat proven tanks in the world, new anti-tank weapons that have yet be used in combat present serious threats to even the M-1 tank. These "new" weapons aren't even new, they have been around for one to three decades and are widely available. The trackbuster mine is a 2-3 pound device that is small, but powerful enough to blow the track off a tank. This will not destroy a tank, but will put the vehicle out of action for hours, or days, until the track is replaced. These mines are cheap and can be delivered by artillery, or placed by hand. Traditional anti-tank mines (what will destroy tanks, but weigh 30-40 pounds) caused 20 percent of American tank losses in Europe during World War II.