Bad Idea? I'm a web dev and maybe some other people could weigh in here on the privacy concern I could imagine from this. Scenario below.

1. you visit site: whatever.com

2. I have a hidden input with a name="address"

3. I have an onchange event listener for the hidden input

4. Now auto complete fills in automatically and I just associated your ip with your address (assuming the the auto fill is correct).

I always think this whenever I hear about auto complete. I NEVER save form data just for this reason.