The guest user account creation is awesome but I think it would make more sense to ask for the desired username and email of the guest instead of the name and email.
This way we can avoid the issue of usernames being generated from the first part of the email which is not what some people want. Since usernames cannot be changed it is important that they choose their own username.
Ajax can be used to check if the username and email address have been used already and should display an error if they are already in use on the site and should instruct the guest to choose another or login if already registered.
There are several other cases of this being requested and I think this makes the most sense from a usability standpoint.