Hi atwoodkevin,
The IFrame mode is the basic upload function.
by default, AjaxUploader will use Silverlight mode for upload. If client does not install silverlight, it will change to Flash mode. If client does not install Flash too, it will change to IFrame mode.
It mean that, all users will not use IFrame mode by default. If he has not silverlight and flash, and you disable the IFrame mode too, and than AjaxUploader will get wrong.
Regards,
Ken