Ask HN: How do you gather email list signups in your web apps?

I want to send interested users email updates about my progress with the app they're signing up to use. I see three choices (fill me in if I'm missing any):

1. Simply add users to the list when they sign up with their email address. Easiest and most effective, but assumes that a signup = permission to send email. Not totally comfortable with this; should I be?

2. Add a checkbox to the signup process that users can check if they want email updates. Nice middle ground, requiring some back-end integration but ensuring I have permission to email. But it won't catch anyone who's already signed up.

3. Add a blog-style pop-up, smartbar, or otherwise prodding UI element to gather signups. Easy for me, but annoying for users who just want to use the app and may already be signed up. But it does catch people who are already signed up for the app and aren't on the list.

All thoughts welcome.