An alternative to doing it in code would be to drop a SQL data source on the page and configure in the following way.
<asp:sqldatasource id="SqlDataSource1" runat="server"
connectionstring="<%$ ConnectionStrings:MyDatabase %>"
selectcommand="SELECT [ID], [First_name] + ' ' + [Last_name] AS [FullName] FROM [tPerson]"></asp:sqldatasource>
And then your dropdown list control:
<asp:dropdownlist id="ddlPeople" runat="server" datasourceid="SqlDataSource1"
datatextfield="FullName" datavaluefield="ID" />
This would all go in the asp.net page.