The N900 has the OMAP3430, which is only capable of a maximum clockrate of 600 Mhz. The architecture of omap3 is capable of 1ghz like the OMAP3640. Big Difference.

The N900 also automatically throttles the clockrate down in order to save battery as well as increasing the clockrate for applications that require more power.

It has been said in the Maemo Summit to NOT increase/lock the clockrate at 600Mhz in order to prevent overheating.