what exactly are you planning to overclock? The CPU is at 2.4GHz, which is the rated speed for that chip. I doubt you will be able to overclock it much higher than that in such a thin laptop without generating unhealthy amounts of heat - it's not like there is a tuniq tower cooler inside or anything. Plus how would you even do it? I highly doubt that apple's EFI "BIOS" allows for FSB overclocking or CPU overvolting.
The only thing you might mean by "downclocking" is the battery management - in both windows and OSX, there are laptop settings that downclock/undervolt the CPU when not at full usage to add more battery life. You can, of course, disable that.
One thing I've heard is that you can overclock the graphics a bit - apple downclocks the graphics core and memory a few MHz from stock speeds - i believe it is 450/650 rather than 475/700 stock. I've heard that using coolbits you can easily bring it up to 500/700, which would help with vista gaming.