I was looking for a simple way to have the convenience of self contained controls, but still use a single validation summary.   I created a base class for user controls so code can be reused without repeating it and a property called ValidationGroup that finds all validation controls in the control and assigns the validation group to them.

public class BaseControl : UserControl
{
    public BaseControl()
    {

    }

    public virtual string ValidationGroup
    {
        set
        {
            foreach (Control control in this.Controls)
            {
                if (control is BaseValidator)
                {
                    ((BaseValidator)control).ValidationGroup = value;
                }
            }
        }
    }
}

Below is an example of a validation summary and a user control called address that finds all controls within it to use the validation group of the validation summary.

<asp:ValidationSummary CssClass="validationSummary" runat="server" ID="ValidationSummary"
    ValidationGroup="vgEvent"  /> 

 <uc:Address ID="CalendarAddress" runat="server" ValidationGroup="vgEvent" />