I'm having the same problem.  I'm trying to add a report to a web project.  I created the DataSet, save it and close it.  When I reopened it, I received the message "Failed to load dataset because of the following error:  Requested value 'Properties' was not found.  The designer then shows the following stack trace:

	One or more errors encountered while loading the designer. The errors are listed below. Some errors can be fixed by rebuilding your project, while others may require code changes.
<div class="ErrorStyle" id="div1" width="100%"> Requested value 'Properties' was not found. Hide    </div>
<div class="StackStyleVisible" id="div20" width="100%"> at System.Enum.Parse(Type enumType, String value, Boolean ignoreCase) at Microsoft.VSDesigner.DataSource.PropertyReferenceSerializer.DeserializeAppConfigExpression(String[] expressionParts) at Microsoft.VSDesigner.DataSource.PropertyReferenceSerializer.Deserialize(String expressionString) at Microsoft.VSDesigner.DataSource.DesignConnection.Microsoft.VSDesigner.DataSource.IDataSourceXmlSpecialOwner.ReadSpecialItem(String propertyName, XmlNode xmlNode, DataSourceXmlSerializer serializer) at Microsoft.VSDesigner.DataSource.DataSourceXmlSerializer.DeserializeBody(XmlElement xmlElement, Object obj) at Microsoft.VSDesigner.DataSource.DataSourceXmlSerializer.Deserialize(XmlElement xmlElement) at Microsoft.VSDesigner.DataSource.DataSourceXmlSerializer.DeserializeBody(XmlElement xmlElement, Object obj) at Microsoft.VSDesigner.DataSource.DesignDataSource.ReadDataSourceExtraInformation(XmlTextReader xmlTextReader) at Microsoft.VSDesigner.DataSource.DataSourceXmlTextReader.Read() at System.Xml.Schema.Parser.Parse(XmlReader reader, String targetNamespace) at System.Xml.Schema.XmlSchema.Read(XmlReader reader, ValidationEventHandler validationEventHandler) at System.Data.DataSet.ReadXSDSchema(XmlReader reader, Boolean denyResolving) at System.Data.DataSet.ReadXmlSchema(XmlReader reader, Boolean denyResolving) at System.Data.DataSet.ReadXmlSchema(XmlReader reader) at Microsoft.VSDesigner.DataSource.DesignDataSource.ReadXmlSchema(DataSourceXmlTextReader xmlReader) at Microsoft.VSDesigner.DataSource.DesignDataSource.ReadXmlSchema(TextReader textReader) at Microsoft.VSDesigner.DataSource.Designer.DataSourceSerializationService.DeserializeToDataSource(String filePath, Object serializationData) at Microsoft.VSDesigner.DataSource.ProjectDataSourceDescriptor.LoadDataSource() at Microsoft.VSDesigner.DataSource.ProjectDataSourceDescriptor.Init(IServiceProvider provider, IVsHierarchy primaryHierarchy, UInt32 primaryItemId, Object primaryDocDataObject, UInt32 docCookie, IVsInvisibleEditor invisibleEditor, IDesignerHost host) at Microsoft.VSDesigner.DataSource.ProjectDataSourceStorage.EnsureInvisibleEditor(ProjectItem dsProjectItem, Boolean ensureWritable, Boolean createInvisibleEditor) at Microsoft.VSDesigner.DataSource.ProjectDataSourceStorage.GetDataSourceInternal(Object caller, ProjectItem dsProjectItem, Boolean ensureWritable, Boolean createInvisibleEditor) at Microsoft.VSDesigner.DataSource.Designer.DataSourceDesignerLoader.HandleLoad(IDesignerSerializationManager serializationManager)</div><div class="StackStyleVisible" id="div20" width="100%"> </div>

I'm running Windows Vista Ultimate Edition and Visual Studio 2005 SP1

Thanks
Chad