As promised earlier this month, the social gaming world’s biggest hit, FarmVille, has gone live on the iPhone. The release is notable because few social game developers have released their games for mobile devices; FarmVille’s developer Zynga, notably, has focused almost entirely on the web since giving up on releasing iPhone versions in the middle of last year.

At the same time, the news may seem a bit boring, since what has reached the iPhone is pretty much the same game that we’ve all known and loved (or hated) for a year: it’s just FarmVille. For now, Zynga is treating the iPhone as a mobile accessory to Facebook, with a core experience that should remain unchanged. The music, art and other key identifiers seem much the same.

That’s not to say there haven’t been changes. They’re just fairly subtle. Other social game developers, though, should be watching carefully to see what Zynga has done with the iPhone’s more limited screen real estate. We set the two versions side by side to get a better look:

There is one major, immediately noticeable change: the farmer character has been removed entirely. This makes sense, given the larger potential for mistakes when using a finger to navigate the game UI. In general, the game is more difficult to use on the iPhone versus using a mouse.

No moving, animated farmer also means less data to load, a point that we’ll return to below.

Another extremely noticeable change is that the empty field in the iPhone version is much more textured than the web version. The new grass ends up looking more like an overgrown meadow than the manicured golf course of FarmVille on Facebook, but it also helps distinguish what you’re looking at on the smaller screen.

Moving on to UI elements, the iPhone version has removed more than it has added. Gone are the sound options; the player can simply change the volume on their iPhone. The currency and point bars remain, but Zynga has moved the “Add Coins & Cash” element, and replaced the player’s name box with a logo in the upper right hand corner that leads to Zynga’s older iPhone titles. Other elements on the side have been removed.

At the bottom of the screen, the cutting got even more intense. The friend bar doesn’t automatically load — it instead scrolls out from the icon on the bottom left. On the bottom right, the various tools, the market, sharing buttons, options and the option to buy virtual currency have all been compressed into a single box, while gifting gets its own box.

That’s mostly it, for the main screen. Other features have visibly changed, like the animal animations, seem as good or better on the iPhone version as compared to the web version.

Some of the changes have probably been inspired by the lower bandwidth of mobile devices, which as we mention above, is probably partially responsible for the farmer character’s loss. The “Loading” icon is a common sight as you use FarmVille on the iPhone, making some parts of the game — like the gifting screen, seen below — less usable.

A final point is that FarmVille’s iPhone version continues a Zynga habit, begun several months ago, of gathering player data for itself rather than working solely through Facebook. When you first sign in, for instance, FarmVille asks for an email address — and doesn’t give an option for Facebook sign-in.

Once your email is entered, the game then offers Facebook access. This is obviously necessary for players to find their old farm and their friends; but when it comes time to hook up with Facebook friends, Zynga makes the option of using your mobile contacts equally prominent.

We’ll keep an eye on both FarmVille’s iPhone and web versions to see how well they do following the release. For now, the web version is down to 63,892,960 players from highs of over 80 million, according to AppData.