The is a persistent problem and an annoying one that is part of the "America is so big that others will just have to accept our defaults" issues. Others include the constant reversion of Outlook to an American spelling dictionary and printers mysteriously adopting "Letter" rather than A4 paper sizes, no matter how many times you tell them otherwise.
I get round this particular issue by always using three-letter month names rather than numbers for the months. This disambiguates the day/month problem, eg: Apr 9 07 is the same as 09 Apr 2007. This always seems to work for data input but the problem is training those pesky users. I use a system that forces them to use three-letter months and will not accept moth numbers so they get used to it in the end