Well, when I read the title of this thread I was all set to launch in, but you have all said it so much more eloquently than I could! I am a teacher, and even amongst my teaching friends I see a big difference. There are the "Oh look she' only three and knows all her letters and numbers" through to the "She didn't know a single letter before she started school, we spent all the time playing" I really think that the most important thing is to be lead by YOUR child. If your child is really in to climbing trees, then that is he/she needs to do. If they are book mad, then read. We really push early reading in this country, with nursery schools now doing letters etc. WHY??????? When I was training, we saw the statistics for Europe. In the UK we start school the earliest. One country (I forget which one) doesn't teach reading until age 8. By age 10, all the children have caught up.
I used to teach in the East End of London, and we saw kids who really had no pre-reading skils. That means they had never seen a book, never heard stories read to them. They had no idea that the black marks on the page told a story, or that the book has a beginning and an end. They were totally at sea when it came to reading, because they didn't understand the concept of stories, writing and books. Those are the children that the government gets worried about, and then imposes early reading on the rest.
Oh dear turned into a bit of a rant, just to repeat the important bit, follow your child, if they need to play, let them play, if they are ready for letters, have fun with them. You will probably find that your child is really tired in the first few months of school, and may not have any enthusiasm for reading at home anyway.
PS, it's probably not a good idea to teach writing letters unless a teacher has shown you how to teach letter formation.