Yup you can hook into the event model and add extra validation rules

Here is how to do it with Contour http://www.nibble.be/?p=430

For Forms it's similar but the controller lives in a different namespace, think it's Umbraco.Forms.Web.Controllers...

It should be similar, it's just the controller that lives in a different namespace so use Umbraco.Forms.Web.Controllers.UmbracoFormsController

@rs82uk, try this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Umbraco.Core;
using Umbraco.Core.Events;
using Umbraco.Core.Logging;
using Umbraco.Core.Models;
using Umbraco.Core.Services;
using Umbraco.Forms;

public class ContourValidation : ApplicationEventHandler
{
    protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        Umbraco.Forms.Web.Controllers.UmbracoFormsController.FormValidate += FormRenderController_FormValidate;

    }

    void FormRenderController_FormValidate(object sender, Umbraco.Forms.Mvc.FormValidationEventArgs e)
    {
        if (e.Form.Name == "FormWithExtraValidation")
        {
            var s = sender as Controller;
            if (s != null)
            {
                var pwf = e.Form.AllFields.SingleOrDefault(f => f.Caption == "Password").Id.ToString();
                var rpwf = e.Form.AllFields.SingleOrDefault(f => f.Caption == "Repeat password").Id.ToString();

                var password = e.Context.Request[pwf];
                var repeatpassword = e.Context.Request[rpwf];

                if (password != repeatpassword)
                    s.ModelState.AddModelError(rpwf, "passwords don’t match");

            }
        }
    }
}

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