Hello Turnipboy,
Your Query is clear, the small difference in the shailaja response is instead of changing the format of text box, change the date format to yyyy-mm-dd in date picker control. InfoPath not allows you to change the textbox format in the properties, you can do it for datepickers.
I hope this will help you better!
B.Shiva Prasad
InfoPath Dev,
http://www.ggktech.com
India