No Miller effect in differential amplifiers
Brian provided a great hint, but here's some more info for reference:

The Miller effect says that if a capacitor is connected between two nodes with a negative voltage gain between them (-A), then the input capacitance looking into the input node will be approximately A times bigger than the capacitance between the nodes.

As a rough analogy think of adding water to a swimming pool with a bucket (That's the input node).  At the same time somebody on the other end is taking water out with a bigger bucket (the node pulling the other end of the capacitor in the opposite direction of your input).  Now you need a lot more water to fill the pool.

In a cascode (bipolar for example) you're using a common base stage to keep the collector voltage of the common emitter stage relatively constant.  So the voltage across the base-collector capacitance isn't changing nearly as much as it would if the collector weren't clamped by the cascade device.  "A" is close to 1.  Therefore the Miller effect is greatly reduced.