Well. We still do only have 256MB of fast RAM unfortunately.

It's not really underclocked. The Cortex A8 when first available would not have had such good yield as they do now. (it was available even back in 2008 if I remember correctly) The specs are still the same however. So 600Mhz is a guaranteed stable speed for mobile devices. Also, they do need to take in considerationthe worst case scenario of heat, power usage etc. I reckon the CPU itself can handle 1ghz just fine in 90% of the samples. It's just that the percentage goes down depending on which environment it is used in. (voltage, heat dissipation etc) If you over volt the cpu, I reckon 1.2ghz+ is easily possible. But, then you are going towards a serious hw-level danger zone.