Marcelo has a nice post detailing how to deal with a couple common cases that can cause 400 responses from a data service.  Check it out here: http://blogs.msdn.com/marcelolr/archive/2008/04/01/fixing-400-bad-request-replies-from-asp-net-when-using-ado-net-data-services.aspx

Mike Flasko

ADO.NET Data Services, Program Manager