"Unless you are posting it via Central Admin - there is no way that .xml would open in the browser via SharePoint. " - my comment was specific to the form that you uploaded to this site for us to review. My company only uses browser based forms, and they are all open in the browser. There is something different about your environment or your process.
You may already know this, but if not, it might help -- The .xml files are based on the .xsn template, so if your template isn't configured/published just right, the.xml files based on that template will not render correctly. If the .xsn is deleted or the person opening the .xml don't have access to the .xsn, it will fail to open or may open just display the xml instead of rendering it in form view.
If you open your .xml files, you will see a reference to the xsn in the header: <?mso-infoPathSolution name="urn:schemas-microsoft-com:office:infopath:MyForm:-myXSD-2010-03-15T15-35-03" solutionVersion="1.0.0.116" productVersion="12.0.0.0" PIVersion="1.0.0.0" href="http://sharepointsite.com/sites/SiteCollection/FormServerTemplates/MyForm.xsn"?>
So more questions... Did you verify in Cental Admin under InfoPath Form services that browser based forms area allowed? Have you been able to get any forms published in sharepoint to open in the browser? Does it work for you and just not other people? You activated the form on the site collection, after that did you add it to a form library? Did you verify in site collection features that the form and SharePoint Enterprise features are both activated?