GAE + Google Accounts API (or OpenId) - Unique Username

Anyone have any ideas on how I could get a unique username (but not the email address) for each user. Ideally, I'd show them a page asking them to specify a username fo开发者_如何学编程r the site, but the only way I can think of doing that is to show it to them after they've authenticated via G Accounts or OpenId. But, that requires another page, and what if they navigate away without entering anything?

As you suggest, you need to ask them after they log in for the first time. Simply redirect them to that page until they fill it out.

Consider seriously, though, if you really need a username. A lot of sites demand one, but then use it for very little - and it's inconvenient for users to have to try and choose a unique one.