I see them do it occasionally. In one recent game I was at war with Hiawatha who was to the south of me along the same coastline. Hiawatha was also at war with Catherine on the other continent. After a small lull in which I didn't see any of his units coming at me by land I spot a decent-sized army in the water (3-4 ships, 6-8 land units including cannons and a GG) with only my caravel to defend! "How clever", I thought, "he's going for a sneak attack against my lightly defended northern part of my empire!" However, this was a force meant for Catherine and I was able to essentially crush it before it even reached her borders. Moral of the story: The AI can and will send the army, but are pretty terrible at making it do anything for them.