I'm going to assume that you have the listbox bound to a datatable. If so, I've had trouble with the SelectedValue myself.
What I've found works is to use the following to get the selected value:
Just replace "Listbox" with the name of your control, and "X" with the index of the selected item you're getting the value for.
If you're only allowing 1 selection at a time, then you can hardcode the "X" as 0.