Adding new fields to the "New User" form

How can we add new/custom fields to the "New User" form (UserNew.aspx)?
At the moment you would have to download the code for the FBA Pack and then modify it in Visual Studio. Under Layouts\FBA\WebParts there's the ascx templates for the ui for the web parts. And then you'd have to modify the web parts themselves to read/save the values for the new fields.
Do you know if there is a way to alter the (MembershipUser) CreateUser function?

If not, where do you recommend saving my new fields to in SharePoint?

You can't modify the CreateUser function directly, as that comes from Microsoft. You could create a wrapper function though, that calls the CreateUser function and does your extra save logic.

I save the new fields to the sharepoint user profile - so just add any fields you need to the sharepoint user profile, and then update the spUser object for the user with the new field values.