Have you tried using a format that is not DATEFORMAT dependent?

The string '29/12/2011 12:00:00 AM' is probably being interpreted as the 12th day of the 29th month 🙂

For example:

'2011-12-29T00:00:00'

'2011-12-29'

'20111229'

See http://msdn.microsoft.com/en-us/library/ms180878.aspx#StringLiteralDateandTimeFormats