I just spent about 15 minutes debugging a problem where a document was getting unexpected nulls where empty strings should have been. Indeed controls like the TextBox have code in them that allows you to set the Text property to null and the TextBox will convert that into an empty string. So it's a bit counterintuitive that the declarative data source works the opposite way by default.

When you use a declarative data source to perform a parameterized update that contains string parameters, consider setting ConvertEmptyStringToNull='false' on your <asp:Parameter> elements, because it's true by default! In other words, if a text field contains an empty string, it'll be sent to your declarative data source not as string.Empty, but as null.

Now I don't know about you, but I don't like dealing with nulls if I can avoid it. Especially strings. Unless there's a clear need to have a null state, I avoid them like the plague not only in my database designs but also in my XML schema designs. Hopefully this helps somebody out!