A little post about backward compatibility with Form Services
Microsoft Office InfoPath 2007 is a tool to both design and fill out electronic forms. Associated with Microsoft Office SharePoint Server 2007 and Forms Service, InfoPath forms will be viewable directly by all supported browsers without any client deployment.
Why Not authoring with “Microsoft InfoPath 2003”?
Because it doesn’t respond to the business need, only version 2007 of Microsoft InfoPath allows the deployment of forms to Form server.
Remark about InfoPath 2003 backward compatibility:
It is possible to design InfoPath 2007 form that is backward compatible with InfoPath 2003 with some limitation. The form must not have .Net code behind as InfoPath 2007 has new object model. Also, Form server does not support InfoPath 2003 scripts or object model.