A bit more background to this:

It was working fine in 1.3.8, prior to the upgrade. Since the upgrade I have re-edited the new english.php files in admin and the catalog (per the FAQ) to the UK date format. As a sanity check, I have checked all of the edits again.

In Catalog -> Featured Products when I pull down the calendar and select a date, it is posted into the date field in the form:

/03/05/2010 (note the dd/mm/yyyy format and the fact that the initial / which is causing the problem).

In Catalog -> Categories/Products if I define or edit a product, pull down the calendar there for "Date Available" and select a date, it is posted into the field in the form:

2010-05-03 (which works fine when I preview and save it).

Regards

Nick