This.
In case people don't understand what it's saying:
Physical storage sizes are measured as 1000MB = 1GB.
Most* computer systems and software reports 1024MB = 1GB.
So when you connect a 8GB device to a computer that reports 1GB as 1024MB, it will say it's capacity is lower than 8GB. This happens with everything, not only iPods and iPhones, but external hard drives and thumb drives. On top of this, the reported space will be even lower once you take into account the operating system.
*I say 'most' because Apple changed Snow Leopard to read 1GB as 1000MB; every version of OS X beforehand, and every version of Windows, reads 1GB as 1024MB. Weirdly, iTunes in Snow Leopard still reads devices as 1024MB per 1GB, unlike the rest of the system.