A seemingly easy thing to do that I could not find documentation for!

string astr;
foreach (DataRowView aRow in listBox1.SelectedItems )
    astr = aRow["TopicId"].ToString();
    astr = aRow["TopicText"].ToString();

TopicId and TopicText are the name of the columns in the table of the Datasource that has been bound to the listbox control.

Drop me a note if you found this useful!