Well SharePoint 2007 supports content types. You could publish from InfoPath 2007 and your users could use 2003. Or, you could try to upload the XSNs from SharePoint - not sure if that will work, but fundamentally SharePoint 2007 supports content types. It has nothing to do with InfoPath except at time of publish. In other words, if you can get the XSNs in SharePoint and configure the content types using SharePoint UI it will work.