Just saw this via steam news: http://store.steampowered.com/news/5596/
Free play of Multiplayer Call of Duty: Black Ops from Thursday 10AM Pacific Time (for us it will start tomorrow-ish) to Sunday (probably 12am pst).
Just go to the store page and click on the free weekend banner to pre-load the game files via Steam (which you can play from 10AM Pacific Time).
Have fun!
so its not a free download