After publishing the form, I could not start filling a form by click the "new" button. It complains that there is not compatible XML editor installaed. I guess you can publish an infopath 2003 form. But, you have to install infopath 2007 client in order to work on it.
You cannot use InfoPath 2003 to publish a browser-enabled form template for Form Services.
However, a user who has the InfoPath 2003 client installed locally should be able to fill out the form if the form template was published using InfoPath 2003. If the form template is published using InfoPath 2007, you have to use Save As to save the form template in InfoPath 2003-compatible format before publishing it; otherwise, InfoPath 2003 users will not be able to open it.