Have a read through this: http://support.microsoft.com/kb/173907?wa=wsignin1.0

The local regional setting has no affect other than how dates are displayed to the user in SQL.

Profiler (running on which machine?) is displaying the date correctly, since May 17, 2013 is the same as 17/05/2013 in DMY format. Goto Tool - Options, Tick "Use regional settings to show date..." and it will be displayed according to the machines Regional setting. I know that's not what you're after, but worth a look to understand display of dates.

The problem may be due to the way the client app converts the date when entered as a char / string. When the character date string is converted to a datetime, it defaults to the Mon dd, yyyy setting. http://msdn.microsoft.com/query/dev10.query?appId=Dev10IDEF1&l=EN-US&k=k%28CAST_TSQL%29;k%28SQL11.SWB.TSQLRESULTS.F1%29;k%28SQL11.SWB.TSQLQUERY.F1%29;k%28MISCELLANEOUSFILESPROJECT%29;k%28DevLang-TSQL%29&rd=true

The client application should have a date picker/calender and state clearly how the parameter should be formatted. EDIT: the client app may be defaulting to mm/dd/yyyy if there is no format specified, however, from what you say, if the app is configured exactly the same on both client machines, then regional settings are the only source which could cause an issue, however that issue is not due to SQL, but rather the client application's handling of date conversion for the parameter.