nice Q.....does this really happen....mite be happening....dude....btw...it is not just the sun thats pulling the plannets.....even the plannets are pulling the sun..(evn they have atteractive forces)...toh...evn tho' not really significant.....it does move a bit...so a movement of sun cud also be affective and cud nullify the change in centre of mass(concidering that the mass of the sun to be pretty high compared to the masses of the plannets...think on it....this is a debatable Q....evn as said by varun...there are other sources that exert external gravitational forces on the solar system.....