"This article describes how to bind a Rich Text Box control on a Microsoft Office InfoPath 2003 form to an XML element that is returned from a Web service."

For more information please see:

http://support.microsoft.com/default.aspx?
    kbid=826996&product=infopath2003

See "Rich Text and Web Services" also:

http://msdn.microsoft.com/library/en-us/ipsdk/html/
    ipsdkRichTextAndWebServices_HV01083592.asp

One more is "You cannot automatically submit structured XML data to a Web service in InfoPath 2003" (applies to InfoPath versions before SP1):

http://support.microsoft.com/default.aspx?
    kbid=826989&product=infopath2003