I'm setting up a new site using 2.4.0 rc1. all is working very well except for the new DateField. I'm modifying an existing template that used CalendarDateField so the new version uses "setConfig('showcalendar')" to achieve the same interface. I'm in Australia so the limitation on showcalendar should not be a problem but ...

The datefield insists on using US style dates m/d/yr rather than d/m/yr. This means that the showcalendar output looks correct (d/m/yr) but fails validation. If I switch to 'dmyfields', the fields are listed in m/d/yr order. If I enter a valid US date manually, the field validates.

In my _config.php I have the line:
i18n::set_locale('en_AU');

I have also added
$dfStart->setLocale('en_AU');
to the page with the DateField

I can run legacydatetimefields and use the old CalendarDateField but the one time I tried, it included the time in the field as well as the date. I may be able to sort that one out but I'd really like to use the new version, if only I can get it to recognise the correct Locale.

Any suggestions as to where to look next to solve this problem?

It appears that i18n is not recognising the locale in the _config.php file. After lots of fruitless effort, I gave up and did a gross hack on i18n.php so get_locale() now says:
return 'en_AU';

The js datepicker doesnt work with other locales (or at least we won't integrate it in core in this capacity). It'll be replaced by a more modern picker, for now the field has the same limitations as 2.3 in this regard. See http://open.silverstripe.org/ticket/5397

It turned out that the problem was that my user account was set to en-US. Once I changed it to en-GB, the datepicker was fine.