Doing some late night research on Wiki I found this:
GLONASS uses a coordinate datum named "PZ-90" (Earth Parameters 1990 - Parametry Zemli 1990), in which the precise location of the North Pole is given as an average of its position from 1900 to 1905. This is in contrast to the GPS's coordinate datum, WGS 84, which uses the location of the North Pole in 1984. As of September 17, 2007 the PZ-90 datum has been updated to differ from WGS 84 by less than 40 cm (16 in) in any given direction.
I could not find any primary reference to quote, but it makes me wonder why in hell the Russians don't just use WGS-84. In any case, WGS-84 is the standard, by ICAO agreement.
GF