I've successfully created an XML Schema and form for editing it. The editing options for the form allow people to add in all kinds of formatting and structures that I don't want at various points.
if you are referring to the rich text editor then yes, you can switch any string type to a plain text editor using an appearance annotation. please refer to the forms developer guide for more information.
please note that there's an unfortunate bug in the plain text editor in 2.0 with respect to loading saved content which has been fixed in 2.0.1 and should be good to go on svn head.