Here is a link to an article:
http://www.hgtv.com/hgtv/rm_walls_maintenance/article/0,1797,HGTV_3803_1393517,00.html
The author recommends a nonshrinking, cement-based filler like "Rockite". He says it to "use a pointed trowel to force the filler into the holes and to scrape excess off with the edge of the trowel".