<asp:BoundField DataField="?????" />
I noticed that the column header said "Item" when auto generated so I tried that, but it didn't work. So then, I ask myself, what is the data field of a System.String? The only bindable member (public field or property) is .Length. Good to know but useless in this case. The ToString() member is a method so it can't be bound to. But apparently there is an undocumented trick: Use the exclamation point (!) as the datafield value and you'll get the string's value.
<asp:BoundField DataField="!" />
This yields the desired result.