Step 1 (after clicking Cancel My Account): "We often find that people cancel for one of the following three reasons. Here's our most persuasive microcopy possible on why that wouldn't be a good idea, two sentences each, with links to longer explanations. [Wow, That's Awesome] [No Thanks, Cancel My Account]
Step 2 (the genius bit): "OK, your account is canceled. Your service will remain active until $DATE. We will happily extend this to $DATE + 7 days and then cancel your account with absolutely no funny business if you take this brief exit survey."