Maybe a username could be made optional, or the same as your email address? For this type of app it's best to keep signup steps absolutely minimal. For instance, is the reCAPTCHA necessary if signing in with a Google account? Those accounts already solved a CAPTCHA when the account was created.
I agree we want it to be as easy as possible. I think removing the captcha for remote authentication is a good idea, we'll look into doing that. I think there is a way to game it with a self hosted openID server, but we'll see if we can't figure out the right way to implement it.
We want to keep usernames, because it reduces spam accounts, and it provides a way for users to address each other anonymously (features coming soon).
Maybe email people reminders with a link in it? Or, consider an integration with Google Latitude so I can friend your site, share my current city, and then it's always updated.
We'll look into Google Latitude. Emailing people near expiration sound like a good idea. Email isn't required though, so we still need to find a good average time.
Thanks for the input!