It appears that you've already received some great advice!
I, too, am a piano and public school music teacher. I would say that readiness definitely varies from child to child. That being said, the youngest student I have worked with was four years old, and in a group music class with two other teachers. The class focused on fostering an appreciation for music and musical exploration using the body, voice, and instruments (small hand percussion). Within the group time, I would take a child for an average of 7 or 8 minutes for individual piano time. During the individual piano time, I taught pre-reading skills (finding the groups of 2 or 3 black keys, etc.).
It really helps if the child knows the alphabet - that way you can start to introduce the white keys by their letter name. You can also work on high/low/same. Singing with the notes helps develop aural skills. Get creative with games, if your child demonstrates an interest.
My oldest son (now almost 3) has been exploring with the piano since he was only 6 months old! I used to put him in his bouncy or exersaucer while I played or practiced, and he would try to reach his arms out to touch the keys. I would sit him on my lap and let him explore. When he began walking, he was just tall enough to reach his arms over his head and push the keys down. Now, he pulls the bench out, and climbs up to play on his own. He loves music, in general, and loves to have Mommy or Daddy (we're both musicians) play one of his favorite songs - like the theme to Little Einsteins on the Disney channel.
There is a music software program that I am a big fan of, called Music Ace. If you go to www.harmonicvision.com, you will find a free demo download to try it out. It has some of those pre reading skills, but gets pretty complex as well. It would be a good tool to use later on for reinforcement of what is learned at the piano. I think it's well worth the investment if your child demonstrates a true interest in learning more about music & piano, as it builds on previously learned skills and concepts.
One of the best things you can do for your child at this age is to simply let him explore, and give him lots of praise for what he discovers both on his own, and with your guidance. Love of music doesn't have to come just from the piano. You can make homemade instruments and find things around the house that make musical sounds, too!
Best of luck!