We require a username and some remote authorization providers don't give one. We just deployed a change so that if a username is available it will show up in the dialogue.
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.
We also made links last for 6 weeks now. This is just for the beta. We'd love to get input as to the best length of time. On the one hand you don't want to have to update it too often, on the other we don't what a bunch of pins on the map for people who've moved on, or aren't using the service anymore.
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.