Inspired by all this I also did a default value data filter that fills a cell with a specified default value whenever the user clears the field.

Here is the code, in case anyone is interested:

'Usage example:
column.Editor.DataFilter = New DefaultValueDataFilter(Of Date)(Today)

'Code:
'(Could loose the Select Case statement for an if statement if you prefer.)
Imports Infragistics.Win
    ''' <summary>
    ''' Use this as UltraGrid column DataFilter when you want to set a default value
    ''' each time the user leaves a cell editor as cleared (e.g. mark the value and
    ''' press delete).
    ''' </summary>
    Public Class DefaultValueDataFilter(Of T)
        Implements IEditorDataFilter

        Private _defaultValue As T

        Public Sub New(defaultValue As T)
            _defaultValue = defaultValue
        End Sub

        Public Function Convert(conversionArgs As EditorDataFilterConvertArgs) As Object Implements IEditorDataFilter.Convert
            Select Case conversionArgs.Direction
                Case ConversionDirection.OwnerToEditor
                    'Do Nothing specific
                Case ConversionDirection.EditorToOwner
                    If IsDBNull(conversionArgs.Value) Then
                        conversionArgs.Handled = True
                        Return _defaultValue
                    End If
            End Select

            Return conversionArgs.Value
        End Function
    End Class

End Namespace

Thanks for the "hack" tip!
I created a generic version of the ZeroToNullDataFilter but where you specify what value is supposed to be handled as null when displayed. This might be useful for example in the case that you can't or don't want to handle nullable types for your class' properties.

Here is the code (in VB.Net) in case anyone is interested:

'Usage example:
column.MaskInput = "{date} {time}"
column.Editor.DataFilter = New ValueToNullDataFilter(Of Date)(New Date(0))
column.NullText = "N/A"

'Code
Imports Infragistics.Win

    ''' <summary>
    ''' When using this DataFilter for a value type, the specified "nullValue" is treated as null in the UltraGrid.
    ''' This enables displaying a specific null text (with the column.NullText property) instead of this "nullValue".
    ''' </summary>
    Public Class ValueToNullDataFilter(Of T As Structure)
        Implements IEditorDataFilter

        Private _nullValue As T

        Public Sub New(nullValue As T)
            _nullValue = nullValue
        End Sub

        Public Function Convert(conversionArgs As EditorDataFilterConvertArgs) As Object Implements IEditorDataFilter.Convert
            Dim value As T? = If(TypeOf (conversionArgs.Value) Is T,
                CType(conversionArgs.Value, T),
                CType(Nothing, T?))

            Select Case conversionArgs.Direction
                Case ConversionDirection.OwnerToEditor
                    If value.HasValue AndAlso value.Value.Equals(_nullValue) Then
                        conversionArgs.Handled = True
                        Return System.DBNull.Value
                    End If
                Case ConversionDirection.EditorToOwner
                    If conversionArgs.Value Is Nothing Then
                        conversionArgs.Handled = True
                        Return _nullValue
                    End If
            End Select
            Return conversionArgs.Value
        End Function
    End Class