Was asked to repost with a better title. So I got the question after playing a lot of Rome:Total War, where War Dogs were used by Romans. They had a few trainers, and several dogs per trainer, who released the dogs to attack the enemies. However, I couldn't find references of Romans ever using them as front line troops(is that the right word?), the only example of dogs being used that way is according to wikipedia,
"Mid-7th century BC: In the war waged by the Ephesians against Magnesia on the Maeander, the Magnesian horsemen were each accompanied by a war dog and a spear-bearing attendant. The dogs were released first and broke the enemy ranks, followed by an assault of spears, then a cavalry charge."
Dogs were certainly used for their superior senses -- Livy (5.47) mentions that the Roman sentinels on the Capitoline during the sack of Rome by the Gauls were accompanied by dogs, who failed to notice a surprise attack during the night (although geese did, leading to the failure of the attack). There is also a reference I recall but am having trouble pinning down to the use of dogs to locate enemies hiding in caves in Sardinia.
As to front-line use -- it's difficult to prove a negative, but I am having extraordinary trouble coming up with any reference to the use of war dogs by the Romans in the way in which the game portrayed them. I would call that with reasonable certainty a gimmick for the purpose of the video game -- like the flaming pigs (which do at least have their root in reality, but it's not as if Roman armies started keeping pigs around across the board to set on fire if elephants showed up) or the anachronistic Pharaonic troops used by the Ptolemies.
Irish Wolfhounds, enormous war dogs used by the Celts, were mentioned by Julius Caesar in his treatise "The Gallic Wars."
In 391 AD Roman Consul Quintus Aurelius Symmachus writes he received seven dogs “canes Scotici” as a gift to be used as war dogs (as well as for fighting lions, bears etc.) and in his words, “all Rome viewed them with wonder”.
I know Pliny the Elder talks about the usage of war mastiffs in his Naturalis Historia (around 80AD), but then that man is pretty much known for making shit up as he goes along.
I also remember a documentary on ancient special forces, which had a section on Roman dog troops. Conveniently, the part has its own Youtube clip and according to them we don't really have detailed Roman accounts of war dog usage but some depictions of battle scenes with them. So I'm eagerly awaiting the arrival of an Ancient Warfare or Roman Antiquity flair for this question.