Ulair, I am pretty sure the computer does peek and cheat on occaision. However, it is greatly toned down in MTW from STW (where it was a great pain). If you save after every turn, occaisionally reloading and trying a different move can reveal that current reinforcements can deterr AI invasions in the same turn while your invasions can prompt AI reinforcements in the same time. However, as other posters said, I think it is necessary to get a decent AI and no longer worries me so much.

One point to notice is that the computer does not seem to cheat when you launch a "Pearl Harbour" style sneak attack. If you launch an unprovoked war in my experience, you can reliably empty your border provinces into your neighbours without worrying about home defence. Once there is open warfare, such a tactic would risk immediate counter-invasion (and in STW, a destructive exchange of provinces).