AJAX :: Cascading Dropdowns After State Is Selected

I have 3 dropdowns in my VB.Net 3.5 web app. After a user selects a value from the first (let's call it Countries) the second (called States) should populate correspondingly. Dropdown three (cities) should also cascade down after a State is selected. The Countries and States dropdowns have SQL DataSources. The Cities dropdown has to call 2 procs (based on the selected State value) so it does not have a SQLDataSource. My question is, the code below works for Countries and States, however, I cannot ge the Cities to populate...

<asp:DropDownList ID="drpCountries" runat="server" DataTextField="CountryName" DataValueField="CountryId" DataSourceID="sdsCountries" AutoPostBack="true">
</asp:DropDownList>
<asp:SqlDataSource ID="sdsCountries" runat="server" ConnectionString="<%&#36; ConnectionStrings:MyConnectionString %>"
SelectCommand="SELECT * FROM [Countries] ORDER BY [CountryName] ASC"></asp:SqlDataSource>
<asp:UpdatePanel ID="updStates" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:DropDownList ID="drpStates" runat="server" DataTextField="StateName" DataValueField="StateId" AutoPostBack="true" DataSourceID="sdsStates">
</asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="drpCountries" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
<asp:SqlDataSource ID="sdsStates" runat="server" ConnectionString="<%&#36; ConnectionStrings:MyConnectionString %>"
SelectCommand="SELECT * FROM [states] WHERE StateId = @StateID ORDER BY [StateName] ASC">
<SelectParameters>
<asp:ControlParameter ControlID="drpCountries" Name="StateID" PropertyName="SelectedValue" DefaultValue="" />
</SelectParameters>
</asp:SqlDataSource>

THIS DOES NOT WORK:

<asp:UpdatePanel ID="updCities" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:DropDownList ID="drpCities" runat="server" DataTextField="CityName" DataValueField="CityId" AutoPostBack="true">
</asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="drpStates" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
Protected Sub drpStates_DataBinding(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles drpStates.DataBinding
' Call some procs and get a DataSet
drpCities.DataSource = ds
drpCities.DataBind()
End Sub