Hi Jo,
Great question! You can use your own schema for your forms and import when you design. The InfoPath namespace is used for XML generated by InfoPath. There are also a couple specific PIs. Try removing the former and opening the XML file. My guess is there will be problems. The PIs will render your XML file InfoPath incompatible. Microsoft has to stick something in the XML file to tell Windows Shell and URLMON that the file is indeed an InfoPath form. That's a reasonable thing. If you look at Word and Excel, you'll see all kinds of special goo in their default file formats. InfoPath is relatively pure compared to those.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra