[nippoo]

Above 400MHz or so, the on-die/on-chip capacitance starts being the most important thing (you're going to have inductance through the legs or balls of the chip).

Putting your decoupling capacitors next to the power pins _does_ cost. Not just in board space, but I've seen and reviewed layouts where the signal traces had to snake around decoupling caps or in some cases through vias because the designer believes that putting the caps close to the pins was the most important thing...