I finally figured out the reason that the test failed in 5763 was because we were inputting a value for the field as value. This isn't a valid date format for the PHP Datetime class (which JDate extends from), therefore we got an exception.
I've replaced it with two tests - one tests when there is a database null value, and the other for when there is a valid string value (this currently fails)
#5611 looks really nice - although I need the change in JHtml::calendar more than the form field (because the two components I'm affected by this with are using FOF :P) but I'm just trying to be consistent across everything.
@wilsonge you tried #5611 ... code changes a little bit similar, but purpose different