I've played FarmVille, and I find it to be not so much a game as a "social engine" -- it's basically a bunch of hooks to keep you "playing" and keep you passing gifts and virtual items between your friends. But it's definitely popular, and having official apps for the iPhone (or perhaps even just sites made to access on the iPhone -- the main game is currently done up in Flash) will likely only increase that popularity.
It'll be interesting to see if Zynga makes its way over to these platforms. Ngmoco has already usurped part of the FarmVille mindshare with its popular We Rule game, but if the real thing shows up, it's unclear whether people will go back to the original, stick with the new, or even play (and spend money on) both. We'll have to see.