If you use this method quit often by passing an `Integer` you will get performance problems because of the boxing and unboxing of the struct.  

My advice would be to either write two methods one using an `Integer` and the other using a `String` as argument, or write a generic method.  

___  

Based on the [naming guidelines][1] you should name methods by using a verb or a verb phrase. `DTableCellValue` looks by its name more as a property than a method.  

Based on the same guidelines method arguments should be named using `camelCase` casing. Although VB.NET isnt case sensitive I would like to suggest to follow these guidelines.  

___  

The `try..catch` is in its current form senseless because if an exception is thrown you are just rethrowing it.  

___  

A construct like  

    If condition Then
        Return Value
    Else
        Return otherValue
    End If  

can be simplified by removing the redundant `else` like so  

    If condition Then
        Return Value
    End If 
    Return otherValue

because if the condition is `true` the `else` won't be reached.  

___  

Implementing the mentioned points (leaving aside the method name) leads to two methods like so  

    Public Function DTableCellValue(ByRef rowData As DataRow, _
                                        ByVal colIndex As Integer, _
                                        Optional ByVal defaultVal As String = "") As Object
        Dim ret As Object = rowData.Item(colIndex)
    
        If IsDBNull(ret) Then
            Return defaultVal
        End If
    
        Return ret
    
    End Function

    Public Function DTableCellValue(ByRef rowData As DataRow, _
                                        ByVal colIndex As String, _
                                        Optional ByVal defaultVal As String = "") As Object
        Dim ret As Object = rowData.Item(colIndex)
    
        If IsDBNull(ret) Then
            Return defaultVal
        End If
    
        Return ret
    
    End Function


  [1]: http://msdn.microsoft.com/en-us/library/ms229002.aspx