Dremel do a grout removal drill bit, that fits within a guide, and Fein do a tool that cuts out grout as well, but be carefull with the depth you go with this one, and check the spaceing between tiles - if 2mm gap, then fine, but if the tiles are fixed edge to edge you will have the added danger of cutting into the back edge of the tile itself, due to the back edge being wider than the front
Simon, Professional tiler
February 2008