Plants vs Zombie is only available on those 3 platforms currently. Since you are already familiar with the PC, I'll go into the other ones. The iPhone version is a bare-bone version of the game, specifically it doesn't have survival mode. It is also missing quite a bit of the bonus games (including the plant garden, the tree of wisdom, as well as the Zombiequarium minigame). It contain the entire campaign, as well as different achievements (also different easter eggs, such as what happen if you scroll down the achievement listing continuously). The puzzles are mainly the same as well.
The iPad version is identical to the iPhone version, except it now includes the survival mode. However, now it includes a new way to improve the power of your cobb plants (by putting butter on enemies first). This is making use of the iPad multitouch mode of up to 11 touches at once (think launching multiple walnut in walnut bowling at once). Still no zen garden.