We've actually had a farming system planned for a long time, I've even made some graphics for different plants growing and stuff. Since then it has kinda taken a backseat since there's so much else in the game we feel take priority over it.
Anyway, the way we've had it setup was pretty straight forward and simple. You would receive seeds during your play through (through quests, enemy drops, chests etc) that you would be able to plant on a little patch at the farm (which is actually in the game already). After a set amount of time, which would vary greatly depending on the seed, different items would grow out of the plant. Maybe not always but most of the times players would have no idea what kind of item the seeds would yield.
The graphics I created was mainly for different pet catching items, but pet feed and even weapons (a leaf sword!) have been something we've wanted to add.
We had talks about having different ways of speeding up the growing process, like watering the plants or whatever. In the end we decided against it, since we didn't want the game to turn into a Harvest Moon. Our vision was pretty much what Own suggested. Something that you could just swing by from time to time that didn't demand any real maintenance from players. The mystery and anticipation about what items you would receive in the end would be enough in my book.
Anyway, we have no real time table for when this system will make it into the game, or if we will end up removing it all together. I still really like the idea, the more side activities the better, but we have so much stuff planned already some of it might end up on the chopping block. Hopefully not though!