Yes it does have such a button. Fn + F8 cycles through the different modes. I havent really needed to use it though – the battery generally seems to last for ever already so on a day to day basis you don’t need to watch what you are doing which is really nice.
I imagine it will come in much more handy if you travelling and away from power for longer periods!
Interestingly/Oddly (I think the latter) Fn+F8 does switch Silent/Normal modes, but Silent doesn’t underclock the processor.
In Samsung Battery Monitor on my NC10, “Max Batt Life” has the CPU Speed set to Minimum and “Normal” has it set to Auto. Yet if you run BattStat and switch between them, you’ll see the CPU stay continually at 1.6GHz in Silent Mode and only drop to 800MHz when it’s idle in Normal.
So Silent Mode switches to the Max Batt Life power profile, this appears to switch off the fan to extend the battery life, but it doesn’t underclock the CPU to save power or heat. So it’s actually generating more heat than it needs to. Explain that…?