Hi Thierry,
I don't think this will be possible with just an XPath. The path is not stored anywhere in the XML. The best I can do for you is give the filepath where the form template is saved. That's in the PIs, so you can access it with XPath. Try adding an expression box to a template with this XPath:
substring-before(substring-after(/processing-instruction()[local-name(.) = "mso-infoPathSolution"], 'href="'), '"')
You can expand to reference anything in the PIs, so you can extract all the information from there, but I don't think the location of the XML file will be there!
PS - Yes, I have replied just last night - check your Inbox in the top right corner of the page. We should continue in regular e-mail to make the notification much easier to see!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge