Julien Phalip: Site-wide, user-configurable date formats for display AND entry
Julien Phalip has put together a fantastic article on how he tackled the problem of different users wanting different date formats.
Using his simple code samples, you could give the user many different choices for date format - yyyy-mm-dd, dd-mm-yy, mm-dd-yyyy, or others. The best part is that these formats are then picked up not only for displaying dates to users, but for any date entry that user has to do on your site.
http://www.julienphalip.com/blog/2008/07/30/simple-site-wide-user-date-format-validation-syste/