Before you fill out the form on Yahoo Mail, you can check whether your username is available. This may seem pointless, because it will check and you can change it when you submit the form, but I think it's a great added incentive to users. It gives them an opportunity to take their time and experiment with different usernames. I think evidence of this is some of my users who will sign up with a username, but then go back and change it slightly. A username check I think could do nothing but increase signups, because of the added flexibility it offers. I might type in a username, know it's available, but then think of something better, because this very process is getting my ideas going. It's the same with keywords in google. Most users don't type in the exact keywords they want. Their behaviour is to try a few, then change them depending on the results. They drill down. To begin with, they didn't know what keywords they wanted. I think understanding this process of username choice and option to change is a good argument for including such a feature in gmail.
Jason
Jason