People seem to forget the root problem behind the 3E Bag of Rats...
The 3E Bag of Rats is built on the combination of cleave being a free attack that can be combined with moves like Whirlwind Attack that can multiple opponents at once. In other words, it is an exploit built upon an ability of the Fighter to abuse 3E's loose regulation of the action economy. In 4E, the action economy is strict. The fact that, in 4E, cleave is a deliberately chosen action that does not stack with attacks that hit multiple targets is a direct move to counter the existence of the Bag of Rats, not further it.
Put simply, if you can't cleave multiple times in a single round in order to hit a single foe more times than you otherwise would be able to, then the Bag of Rats is impossible. It will always be better to use an attack actually designed to hurt a single foe.
As FireLance mentioned above, compare the "broken" use of 4E cleave to another power we are vaguely aware of: the ability to do damage equal to your strength modifier regardless of whether you hit or not. If there are minions around the boss, cleave works to do a minor chance of damage (but not guaranteed, since you need to hit the minion), but if not, you can attack the boss without any chance of not doing damage and also have a chance of doing full damage or critical damage. Also, keep in mind that 4E seems to presuppose that you will always have a reasonable chance of hitting your foe. 4E does not want characters to rely upon autohit attacks in order to be useful (look at Magic Missile), it wants you to always have a fair chance.
I really don't see any problem at all with the new cleave, given what we know about 4E's more fundamental changes.