[Master Belisarius]

Yes, the AI use the cloak armor, but depend who the research tree was done for the AI creator. But if the cloak device is available, always will use it instead the cloak armor.
To see a race that use cloak, try the Pyrochette.
The problem with the cloak, is that currently cloaked ships can't use their minesweepers to remove mines... and then, the AI will lose entire cloaked fleets, altough these ships could have minesweepers devices included.