Try this:
<asp:Repeater ID="myRepeater" runat="server">
<ItemTemplate>
<td>
<asp:CheckBox runat="server" ID="cb" Checked='<%# Container.DataItem %>' />
</td>
</ItemTemplate>
</asp:Repeater>
Eval(x) is a shortcut for Databinder.Eval(Container.DataItem, x). Which evalutes the property/etc x for the row item in the repeater. But you don't need to evaluate anything, you just want the raw DataItem.