Have you tried to turn on SharePoint history? Change tracking isn't supported by InfoPath. But, you can store history in SharePoint (MOSS 2007). However, to verify a change, you'll need to query previous doc on submit and diff it yourself. Too bad InfoPath didn't support this. Of course, you could also use our Web Service suite (DBXL) - we support full history, auditing, and we are HIPAA compliant .... but I'm pretty sure you are just looking fore free help, right? :)