I think it's surprising any time a baseball team outright releases a player, especially one of Giambi's name and reputation, but it's not a shock in terms of production. Giambi played in 83 games for the A's this season and was last in the league in batting average.
The question now is if anyone will sign him for the stretch run.
My guess is that he doesn't end up with anyone. An American League team is the only logical fit but many of the contending teams don't have room for him. Giambi could perhaps catch on as a pinch hitter, but with a .193 average, teams aren't exactly going to be knocking down his door.