Hi Dave,
After installing the form, don't use the xsn. Use the form from the Fill Out a Form dialog. Just Start InfoPath and choose the registered form. You may need to remove the form first and rerun the installation script. Anyway, the key is not to click on the xsn that you sent to the tester but to use the one that is cached in InfoPath. Good luck!
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra