one problem as i remember it is that the meter is defined as the distance that light travelsin a certain fixed period of time. problems here are that the speed of light are not constant and also that time is also not constant and depends on the observers point of view. i believe that these issues were resolved using very precise definations of time observed, speed of light etc but it took years and i can only presume that it will also take years to get the kilogram standardised.
Cheers
~M

>http://www.physicsweb.org/article/news/7/9/9
>
>From the seven basic SI-units, the kilogram is the only one that is not yet defined in terms of fundamental constants. Instead, it is defined as the mass of a prototype. Scientists want to change this.
>
>(For those who are not familiar with the SI-system: the SI-system is the international standard for units of measure. It has seven base units:
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>Kilogram for mass
>Meter for length
>Second for time
>Ampère for electrical current
>Mole for amount of substance
>Kelvin for temperature
>
>and a unit for brilliance, IIRC.
>
>All other units are derived from the base units; for instance, speed is measured in meters/second.)