By the way, I'm using the 20229 version of the toolkit. Here's the code.
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<ajaxToolkit:MaskedEditExtender ID="MaskedEditExtender1"
runat="server"
TargetControlID="TextBox1"
Mask="99/99/9999"
MaskType=Date
MessageValidatorTip=true />