Hi Jay,
Thanks for the blow-by-blow account of what happened. Without looking at the form, I have no idea what might have caused this. Sorry, but my ESP crystals aren't vibrating tonight. Maybe I'll switch to cards and try a hand of Tarot... :) Seriously, I'm glad you are back in action. Try diffing the xsns by extracting the form contents (File->Extract Forms) and comparing with a tool like windiff. Cheers!
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra